Pregnancy is a period of up to 41 weeks in which a fetus develops inside a woman’s womb. NICHD conducts and supports research and training to help promote healthy pregnancies, with a focus on the important events that occur before, during, and after pregnancy.

About Pregnancy

Pregnancy is the term used to describe the period in which a fetus develops inside a woman's womb or uterus.

Pregnancy usually lasts about 40 weeks, or just over 9 months, as measured from the last menstrual period to delivery. Health care providers refer to three segments of pregnancy, called trimesters. The major events in each trimester are described below.1

The events that lead to pregnancy begin with conception, in which a sperm penetrates an egg. The fertilized egg (called a zygote) then travels through the woman's fallopian tube to the uterus, where it implants itself in the uterine wall. The zygote is made up of a cluster of cells that later form the fetus and the placenta. The placenta connects the mother to the fetus and provides nutrients and oxygen to the fetus.2

  • Between 18 and 20 weeks, the typical timing for ultrasound to look for birth defects, you can often find out the sex of your baby.
  • At 20 weeks, a woman may begin to feel movement.
  • At 24 weeks, footprints and fingerprints have formed and the fetus sleeps and wakes regularly.
  • According to research from the NICHD Neonatal Research Network, the survival rate for babies born at 28 weeks was 92%, although those born at this time will likely still experience serious health complications, including respiratory and neurologic problems.3

  • At 32 weeks, the bones are soft and yet almost fully formed, and the eyes can open and close.
  • Infants born before 37 weeks are considered preterm. These children are at increased risk for problems such as developmental delays, vision and hearing problems, and cerebral palsy.4 Infants born between 34 and 36 weeks of pregnancy are considered to be "late preterm."4
  • Infants born in the 37th and 38th weeks of pregnancy—previously considered term—are now considered "early term." These infants face more health risks than infants who are born at 39 weeks or later, which is now considered full term.6
  • Infants born at 39 or 40 weeks of pregnancy are considered full term. Full-term infants have better health outcomes than do infants born earlier or, in some cases, later than this period. Therefore, if there is no medical reason to deliver earlier, it is best to deliver at or after 39 weeks to give the infant's lungs, brain, and liver time to fully develop.6,7,8
  • Infants born at 41 weeks through 41 weeks and 6 days are considered late term.6
  • Infants who are born at 42 weeks and beyond are considered post term.6


  1. Office on Women's Health. (2010). Stages of pregnancy. Retrieved May 20, 2016, from
  2. American College of Obstetricians and Gynecologists (ACOG). (2020). Patient education: How your fetus grows during pregnancy. Retrieved December 30, 2020, from external link
  3. Stoll, B. J., Hansen, N. I., Bell, E. F., Shankaran, S., Laptook, A. R., Walsh, M. C., et al. (2010). Neonatal outcomes of extremely preterm infants from the NICHD Neonatal Research Network. Pediatrics, 126, 443–456. PMID: 20732945
  4. Spong, C. Y. (2013). Defining "term" pregnancy: Recommendations from the Defining "Term" Pregnancy Workgroup. JAMA, 309(13), 2445–2446. Retrieved October 28, 2013, from external link
  5. Centers for Disease Control and Prevention. (n.d.). CDC WONDER: About natality, 2007-2014. Retrieved May 20, 2016, from
  6. ACOG Committee on Obstetric Practice and Society for Maternal-Fetal Medicine. (2013; Reaffirmed 2015). Committee Opinion No. 579. Definition of term pregnancy. Retrieved May 20, 2016, from external link
  7. NICHD. (2013). Carrying pregnancy to 39 weeks: Is it worth it? Yes! Retrieved July 8, 2016, from
  8. NICHD. (2013). Redefining the term. Retrieved July 8, 2016, from

What are some common signs of pregnancy?

The primary sign of pregnancy is missing a menstrual period or two or more consecutive periods, but many women experience other symptoms of pregnancy before they miss a period.

Missing a period does not always mean a woman is pregnant. Menstrual irregularities are common and can have a variety of causes, including taking birth control pills, conditions such as diabetes and polycystic ovary syndrome, eating disorders, and certain medications. Women who miss a period should see their health care provider to find out whether they are pregnant or whether they have another health problem.

Pregnancy symptoms vary from woman to woman. A woman may experience every common symptom, just a few, or none at all. Some signs of early pregnancy include:1

8 other signs you might be pregnant
  • Slight bleeding. One study shows as many as 25% of pregnant women experience slight bleeding or spotting that is lighter in color than normal menstrual blood.2 This typically occurs at the time of implantation of the fertilized egg (about 6 to 12 days after conception) but is common in the first 12 weeks of pregnancy.3
  • Tender, swollen breasts or nipples. Women may notice this symptom as early as 1 to 2 weeks after conception. Hormonal changes can make the breasts sore or even tingly. The breasts feel fuller or heavier as well.1
  • Fatigue. Many women feel more tired early in pregnancy because their bodies are producing more of a hormone called progesterone, which helps maintain the pregnancy and encourages the growth of milk-producing glands in the breasts. In addition, during pregnancy the body pumps more blood to carry nutrients to the fetus. Pregnant women may notice fatigue as early as 1 week after conception.4
  • Headaches. The sudden rise of hormones may trigger headaches early in pregnancy.4
  • Nausea and/or vomiting. This symptom can start anywhere from 2 to 8 weeks after conception and can continue throughout pregnancy. Commonly referred to as "morning sickness," it can actually occur at any time during the day.1
  • Food cravings or aversions. Sudden cravings or developing a dislike of favorite foods are both common throughout pregnancy. A food craving or aversion can last the entire pregnancy or vary throughout this period.1
  • Mood swings. Hormonal changes during pregnancy often cause sharp mood swings. These can occur as early as a few weeks after conception.5
  • Frequent urination. The need to empty the bladder more often is common throughout pregnancy. In the first few weeks of pregnancy, the body produces a hormone called human chorionic gonadotropin, which increases blood flow to the pelvic region, causing women to have to urinate more often.4

Many of these symptoms can also be signs of other conditions, the result of changing birth control pills, or effects of stress, so they do not always mean that a woman is pregnant. Women should see their health care provider if they suspect they are pregnant.


  1. American Pregnancy Association. (2015). Pregnancy symptoms—Early signs of pregnancy. Retrieved May 20, 2016, from external link  
  2. Deutchman, M., Tubay, A. T., & Turok, D. (2009). First trimester bleeding. American Family Physician, 79, 985–994. PMID: 19514695
  3. American College of Obstetricians and Gynecologists. (2019). Bleeding during pregnancy. FAQ038. Retrieved December 30, 2020, from external link
  4. March of Dimes. (2013). Common discomforts of pregnancy. Retrieved March 10, 2020, from external link 
  5. American Pregnancy Association. (2015). Mood swings during pregnancy. Retrieved May 20, 2016, from external link 

How do I know if I’m pregnant?

If you have missed one or more menstrual periods or have one or more of the early signs of pregnancy, you may wonder whether you are pregnant.

Home pregnancy tests, which are highly accurate and available without a prescription, can be the first way women determine if they are pregnant. If a home pregnancy test is positive, a woman should call her health care provider to schedule an appointment.

Home pregnancy tests measure the amount of human chorionic gonadotropin (hCG) in a woman's urine. Small amounts of this hormone are present even before the first missed period, and they increase as pregnancy continues.

NICHD research in the 1970s led to the development of the home pregnancy test. Researchers were studying the role of hCG in tracking the success of a cancer treatment. During this study, researchers discovered that high levels of hCG in the urine were associated with pregnancy. Manufacturers used this research to create home pregnancy tests that detect hCG levels in urine with up to 97% accuracy.

What is prenatal care and why is it important?

Having a healthy pregnancy is one of the best ways to promote a healthy birth. Getting early and regular prenatal care improves the chances of a healthy pregnancy. This care can begin even before pregnancy with a pre-pregnancy care visit to a health care provider.

A pre-pregnancy care visit can help women take steps toward a healthy pregnancy before they even get pregnant.
Women can help to promote a healthy pregnancy and birth of a healthy baby by taking the following steps before they become pregnant:1

  • Develop a plan for their reproductive life.
  • Increase their daily intake of folic acid (one of the B vitamins) to at least 400 micrograms.2
  • Make sure their immunizations are up to date.
  • Control diabetes and other medical conditions.
  • Avoid smoking, drinking alcohol, and using drugs.
  • Attain a healthy weight.
  • Learn about their family health history and that of their partner.
  • Seek help for depression, anxiety, or other mental health issues.

Women who suspect they may be pregnant should schedule a visit to their health care provider to begin prenatal care. Prenatal visits to a health care provider usually include a physical exam, weight checks, and providing a urine sample. Depending on the stage of the pregnancy, health care providers may also do blood tests and imaging tests, such as ultrasound exams. These visits also include discussions about the mother's health, the fetus's health, and any questions about the pregnancy.3

Pre-Pregnancy and prenatal care can help prevent complications and inform women about important steps they can take to protect their infant and ensure a healthy pregnancy. With regular prenatal care women can:

  • Reduce the risk of pregnancy complications. Following a healthy, safe diet; getting regular exercise as advised by a health care provider; and avoiding exposure to potentially harmful substances such as lead and radiation can help reduce the risk for problems during pregnancy and promote fetal health and development.4 Controlling existing conditions, such as high blood pressure and diabetes, is important to prevent serious complications and their effects.5
  • Reduce the fetus's and infant's risk for complications. Tobacco smoke and alcohol use during pregnancy have been shown to increase the risk for Sudden Infant Death Syndrome.6 Alcohol use also increases the risk for fetal alcohol spectrum disorders, which can cause a variety of problems such as abnormal facial features, having a small head, poor coordination, poor memory, intellectual disability, and problems with the heart, kidneys, or bones.7 According to one recent study supported by the NIH, these and other long-term problems can occur even with low levels of prenatal alcohol exposure.8

    In addition, taking 400 micrograms of folic acid daily reduces the risk for neural tube defects by 70%.2,9 Most prenatal vitamins contain the recommended 400 micrograms of folic acid as well as other vitamins that pregnant women and their developing fetus need.1,10 Folic acid has been added to foods like cereals, breads, pasta, and other grain-based foods. Although a related form (called folate) is present in orange juice and leafy, green vegetables (such as kale and spinach), folate is not absorbed as well as folic acid.
  • Help ensure the medications women take are safe. Women should not take certain medications, including some acne treatments11 and dietary and herbal supplements,12 during pregnancy because they can harm the fetus.

Learn more about prenatal and pre-pregnancy care.


  1. Centers for Disease Control and Prevention. (2015). Preconception health and health care. Retrieved May 20, 2016, from
  2. U.S. Preventive Services Task Force (2017). Final recommendation statement: Folic acid for the prevention of neural tube defects: Preventive medication. Retrieved January 17, 2017, from
    external link
  3. March of Dimes. (2011). Your first prenatal care checkup. Retrieved May 20, 2016, from external link
  4. Child Trends Databank. (2015). Late or no prenatal care. Retrieved May 20, 2016, from external link
  5. American College of Obstetricians and Gynecologists. (2014). Preeclampsia and high blood pressure during pregnancy. FAQ034. Retrieved May 20, 2016, from external link
  6. American College of Obstetricians and Gynecologists. (2013). Tobacco, alcohol, drugs, and pregnancy. FAQ170. Retrieved May 20, 2016, from external link
  7. Centers for Disease Control and Prevention. (2011). Fetal alcohol spectrum disorders. Retrieved August 1, 2012, from
  8. Eckstrand, K. L., Ding, Z., Dodge, N. C., Cowan, R. L., Jacobson, J. L., Jacobson, S. W., et al. (2012). Persistent dose-dependent changes in brain structure in young adults with low-to-moderate alcohol exposure in utero. Alcoholism: Clinical and Experimental Research, 36(11), 1892–1902. PMID: 22594302
  9. Centers for Disease Control and Prevention. (2016). Folic acid. Data and statistics. Retrieved December 12, 2016, from
  10. NIH Office of Dietary Supplements. (2016). Folate.Dietary supplement fact sheet. Retrieved May 20, 2016, from 
  11. American Pregnancy Association. (2015). Acne during pregnancy. Retrieved May 20, 2016, from external link
  12. Office on Women's Health. (2012). Prenatal care fact sheet. Retrieved May 20, 2016, from

What are some common complications of pregnancy?

Some women experience health problems during pregnancy. These complications can involve the mother's health, the fetus's health, or both. Even women who were healthy before getting pregnant can experience complications. These complications may make the pregnancy a high-risk pregnancy.

For the latest information on COVID-19 and pregnancy, visit CDC at

Getting early and regular prenatal care can help decrease the risk for problems by enabling health care providers to diagnose, treat, or manage conditions before they become serious. Prenatal care can also help identify mental health concerns related to pregnancy, such as anxiety and depression.

Some common complications of pregnancy include, but are not limited to, the following.

High blood pressure, also called hypertension, occurs when arteries carrying blood from the heart to the body organs are narrowed. This causes pressure to increase in the arteries. In pregnancy, this can make it hard for blood to reach the placenta, which provides nutrients and oxygen to the fetus.1 Reduced blood flow can slow the growth of the fetus and place the mother at greater risk of preterm labor and preeclampsia.1,2

Women who have high blood pressure before they get pregnant will continue to have to monitor and control it, with medications if necessary, throughout their pregnancy. High blood pressure that develops in pregnancy is called gestational hypertension. Typically, gestational hypertension occurs during the second half of pregnancy and goes away after delivery.

Gestational diabetes occurs when a woman who didn't have diabetes before pregnancy develops the condition during pregnancy.

Normally, the body digests parts of your food into a sugar called glucose. Glucose is your body's main source of energy. After digestion, the glucose moves into your blood to give your body energy.

To get the glucose out of your blood and into the cells of your body, your pancreas makes a hormone called insulin. In gestational diabetes, hormonal changes from pregnancy cause the body to either not make enough insulin, or not use it normally. Instead, the glucose builds up in your blood, causing diabetes, otherwise known as high blood sugar.

Managing gestational diabetes, by following a treatment plan outlined by a health care provider, is the best way to reduce or prevent problems associated with high blood sugar during pregnancy. If not controlled, it can lead to high blood pressure from preeclampsia and having a large infant, which increases the risk for cesarean delivery.4

Infections, including some sexually transmitted infections (STIs), may occur during pregnancy and/or delivery and may lead to complications for the pregnant woman, the pregnancy, and the baby after delivery. Some infections can pass from mother to infant during delivery when the infant passes through the birth canal; other infections can infect a fetus during the pregnancy.1 Many of these infections can be prevented or treated with appropriate pre-pregnancy, prenatal, and postpartum follow-up care.

Some infections in pregnancy can cause or contribute to:1

  • Pregnancy loss/miscarriage (before 20 weeks of pregnancy)
  • Ectopic pregnancy (when the embryo implants outside of the uterus, usually in a fallopian tube)
  • Preterm labor and delivery (before 37 completed weeks of pregnancy)
  • Low birth weight
  • Birth defects, including blindness, deafness, bone deformities, and intellectual disability
  • Stillbirth (at or after 20 weeks of pregnancy)
  • Illness in the newborn period (first month of life)
  • Newborn death
  • Maternal health complications

If you are planning to get pregnant, talk with your health care provider about getting vaccines and vaccine boosters for chicken pox (also called varicella) and rubella (also called German measles) before you conceive. You can also get some vaccines, such as the flu shot, while you are pregnant. If you know you have an infection, such as an STI, talk with your health care provider about it before you conceive to increase your chances of a healthy pregnancy.

Early prenatal testing for STIs and other infections can determine if the infection can be cured with drug treatment. Or, if you know you have an infection, tell your pregnancy health care provider about it as early as possible in your pregnancy. Early treatment decreases the risk to the fetus and infant.2 Even if the infection can't be cured, you and your health care provider can take steps to protect your health and your infant's health.

Learn more about infections that can affect pregnancy.

Preeclampsia is a serious medical condition that can lead to preterm delivery and death. Its cause is unknown, but some women are at an increased risk. Risk factors include:5

  • First pregnancies5
  • Preeclampsia in a previous pregnancy5
  • Existing conditions such as high blood pressure, diabetes, kidney disease, and systemic lupus erythematosus1
  • Being 35 years of age or older1
  • Carrying two or more fetuses1
  • Obesity1

Preterm labor is labor that begins before 37 weeks of pregnancy. Any infant born before 37 weeks is at an increased risk for health problems, in most cases because organs such as the lungs and brain finish their development in the final weeks before a full-term delivery (39 to 40 weeks).

Certain conditions increase the risk for preterm labor, including infections, developing a shortened cervix, or previous preterm births.6

Progesterone, a hormone produced naturally during pregnancy, may be used to help prevent preterm birth in certain women. A 2003 study led by NICHD researchers found that progesterone supplementation to women at high risk for preterm delivery due to a prior preterm birth reduces the risk of a subsequent preterm birth by one third.7

Research shows that as many as 13% of U.S. women reported frequent symptoms of depression after childbirth, and that anxiety co-occurs in up to 43% of depressed pregnant and postpartum women, making pregnancy-related depression and anxiety among the more common pregnancy complications.8 These medical conditions can have significant effects on the health of the mother and her child. But the good news is that these are treatable medical conditions. The NICHD-led Moms’ Mental Health Matters initiative is designed to educate families and health care providers about who is at risk for depression and anxiety during and after pregnancy, the signs of these problems, and how to get help. 

Miscarriage is the term used to describe a pregnancy loss from natural causes before 20 weeks. Signs can include vaginal spotting or bleeding, cramping, or fluid or tissue passing from the vagina. However, bleeding from the vagina does not mean that a miscarriage will happen or is happening.9 Women experiencing this sign at any point in their pregnancy should contact their health care provider.

The loss of pregnancy after the 20th week of pregnancy is called a stillbirth. In approximately half of all reported cases, health care providers can find no cause for the loss. However, health conditions that can contribute to stillbirth include chromosomal abnormalities, placental problems, poor fetal growth, chronic health issues of the mother, and infection. Read more about health conditions and lifestyle factors that can increase the risk for stillbirth.

Other complications of pregnancy may include the following:

  • Severe, persistent nausea and vomiting. Although having some nausea and vomiting is normal during pregnancy, particularly in the first trimester, some women experience more severe symptoms that last into the third trimester.

    The cause of the more severe form of this problem, known as hyperemesis gravidarum (pronounced HEYE-pur-EM-uh-suhss grav-uh-DAR-uhm), is not known. Women with hyperemesis gravidarum experience nausea that does not go away, weight loss, reduced appetite, dehydration, and feeling faint.10

    Affected women may need to be hospitalized so that they can receive fluids and nutrients. Some women feel better after their 20th week of pregnancy, while others experience the symptoms throughout their pregnancy.10
  • Iron-deficiency anemia. Pregnant women need more iron than normal for the increased amount of blood they produce during pregnancy. Iron-deficiency anemia—when the body doesn't have enough iron—is somewhat common during pregnancy and is associated with preterm birth and low birth weight. Symptoms of a deficiency in iron include feeling tired or faint, experiencing shortness of breath, and becoming pale. ACOG recommends 27 milligrams of iron daily (found in most prenatal vitamins) to reduce the risk for iron-deficiency anemia. Some women may need extra iron through iron supplements.12 Your health care provider may screen you for iron-deficiency anemia and, if you have it, may recommend iron supplements.13


  1. American College of Obstetricians and Gynecologists. (2020). Preeclampsia and high blood pressure during pregnancy. FAQ034. Retrieved December 30, 2020, external link
  2. Leeman, L., & Fontaine, P. (2008). Hypertensive disorders of pregnancy. American Family Physician, 78, 93–100. PMID: 18649616
  3. Centers for Disease Control and Prevention. (2015). Births: Final data for 2014. Supplemental table I-6. National Vital Statistics Report, 64(12). Retrieved May 31, 2016, from (PDF 867 KB)
  4. ACOG. (2013). Gestational diabetes. FAQ177. Retrieved May 31, 2016, from external link
  5. Hernandez-Diaz, S., Toh, S., & Cnattinguis, S. (2009). Risk of pre-eclampsia in first and subsequent pregnancies: prospective cohort study. British Medical Journal, 338, b2255. Retrieved July 31, 2012, from
  6. Office on Women's Health. (2010). Pregnancy: pregnancy complications. Retrieved May 31, 2016, from
  7. Meis, P. J., Klebanoff, M., Thom E., Dombrowski, M. P., Sibai, B., Moawad, A. H., et al. (2003). Prevention of recurrent preterm delivery by 17 alpha-hydroxyprogesterone caproate. New England Journal of Medicine, 348, 2379–2385. PMID: 12802023
  8. Le Strat, Y., Dubertret, C., & Le Foll, B. (2011). Prevalence and correlates of major depressive episode in pregnant and postpartum women in the United States. Journal Of Affective Disorders, 135(1-3), 128-138. doi: 10.1016/j.jad.2011.07.004.
  9. ACOG. (2015). Early pregnancy loss. FAQ090. Retrieved May 31, 2016, from external link
  10. National Organization for Rare Diseases. (2015). Hyperemesis gravidarum. Retrieved May 31, 2016, from external link
  11. ACOG. (2016). Routine tests during pregnancy. FAQ133. Retrieved August 29, 2016, from external link
  12. ACOG. (2015). Nutrition during pregnancy. FAQ001. Retrieved May 31, 2016, from external link
  13. Allen, L. H. (2000). Anemia and iron deficiency: effects on pregnancy outcome. American Journal of Clinical Nutrition, 71(5), 1280s–1284s. Retrieved November 9, 2016, from external link

What is a high-risk pregnancy?

high-risk pregnancy is one that threatens the health or life of the mother or her fetus. It often requires specialized care from specially trained providers.

Some pregnancies become high risk as they progress, while some women are at increased risk for complications even before they get pregnant for a variety of reasons.

Early and regular prenatal care helps many women have healthy pregnancies and deliveries without complications.

Risk factors for a high-risk pregnancy can include:

  • Existing health conditions, such as high blood pressure, diabetes, or being HIV-positive1
  • Overweight and obesity. Obesity increases the risk for high blood pressure, preeclampsia, gestational diabetes, stillbirth, neural tube defects, and cesarean delivery. NICHD researchers have found that obesity can raise infants' risk of heart problems at birth by 15%.3
  • Multiple births. The risk of complications is higher in women carrying more than one fetus (twins and higher-order multiples). Common complications include preeclampsia, premature labor, and preterm birth. More than one-half of all twins and as many as 93% of triplets are born at less than 37 weeks' gestation.4
  • Young or old maternal age. Pregnancy in teens and women age 35 or older increases the risk for preeclampsia and gestational high blood pressure.5,6

Women with high-risk pregnancies should receive care from a special team of health care providers to ensure the best possible outcomes.

For more information, visit the High-Risk Pregnancy topic.


  1. American College of Obstetricians and Gynecologists. (2012). HIV and pregnancy. FAQ113. Retrieved May 31, 2016, from external link
  2. American College of Obstetricians and Gynecologists. (2015). ACOG Practice Bulletin No. 156: Obesity in pregnancy. Obstetrics and Gynecology, 126(6), e112–126. PMID: 26595582
  3. NIH. (2010). Risk of newborn heart defects increases with maternal obesity [news release]. Retrieved July 30, 2012, from
  4. Hamilton, B. E., Martin, J. A., Osterman, M. J. K., Curtin, S. C., & Mathews, T. J. (2015). Births: Final data for 2014. National Vital Statistics Reports, 64(12). Retrieved May 31, 2016, from (PDF – 2.95 MB)
  5. MedlinePlus. (2011). Medical Encyclopedia: Adolescent pregnancy. Retrieved May 31, 2016, from
  6. MedlinePlus. (2014). Medical Encyclopedia: preeclampsia. Retrieved May 31, 2016, from

What infections can affect pregnancy?

Which infections can affect pregnancy?For the latest information on COVID-19 and pregnancy, visit CDC at

Infections that can affect the health of the pregnant woman, the pregnancy, and the baby after delivery include (but are not limited to):

  • Bacterial vaginosis is the most common vaginal infection in women of reproductive age. It increases the risk of contracting sexually transmitted infections (STIs) and may play a role in preterm labor. The condition results from a change in the balance of bacteria that normally live in the vagina. Having unprotected sex and douching can increase the risk of bacterial vaginosis. The Centers for Disease Control and Prevention (CDC) recommends that pregnant women get tested for bacterial vaginosis if they have symptoms and get treated if necessary.1
  • Chlamydia infection during pregnancy is associated with an increased risk of preterm birth and its complications.2 If the infection is present and untreated at the time of delivery, it can lead to eye infections or pneumonia in the infant.1 In most hospitals, infants' eyes are routinely treated with an antibiotic ointment shortly after birth. The ointment can prevent blindness from exposure to chlamydia bacteria during delivery in case the pregnant woman had an undetected infection.
  • Cytomegalovirus (CMV) is a common virus present in many body fluids that can be spread through close personal contact, such as kissing or sharing eating utensils, as well as sexual contact. The virus usually does not cause health problems, but once it is in a person's body, it stays there for life and can reactivate at different times. A pregnant woman may not even know she has the infection, and she may pass the virus on to her fetus, causing congenital CMV infection. Most infants with congenital CMV infection never show signs or have health problems. However, some infants have health problems such as hearing or vision loss, seizures, or intellectual disabilities that are apparent at birth or that develop later during infancy or childhood.3 Currently, routine screening for CMV during pregnancy is not recommended. Researchers are working on treatments for CMV and vaccines to try to prevent new infections during pregnancy and to reduce the risk of transmission to the infant.4Congenital CMV infection can be diagnosed by testing a newborn baby's saliva, urine, or blood. Treatment with antiviral drugs may decrease the risk of health problems and hearing loss in some infected infants.
  • Fifth disease is caused by human parvovirus type B19. The virus causes a common childhood disease that spreads easily from person to person. Children who get it usually have a fever and a red rash on their cheeks. Parvovirus B19 usually does not cause problems for pregnant women or the fetus, but in rare cases, the woman might have a miscarriage or the fetus could develop anemia. There is no vaccine or treatment for fifth disease. You can reduce your chance of being infected with parvovirus B19 or infecting others by avoiding contact with people who have parvovirus B19 and by thoroughly and regularly washing your hands. Sometimes health care providers recommend testing pregnant women to see if they are immune to the virus already.
  • Untreated gonorrhea infection in pregnancy has been linked to miscarriage, preterm birth and low birth weight, premature rupture of the membranes surrounding the fetus in the uterus, and infection of the fluid that surrounds the fetus during pregnancy. Gonorrhea can also infect an infant during delivery as it passes through the birth canal. If untreated, infants can develop eye infections and blindness. In most hospitals, infants' eyes are routinely treated with an antibiotic ointment shortly after birth to prevent eye problems from exposure to gonorrhea during delivery, in case the pregnant woman had an undetected infection. Treating gonorrhea as soon as it is detected in pregnant women reduces the risk of transmission.
  • Group B streptococcus (GBS) can cause serious health problems in infants. But giving antibiotics during labor can prevent the spread of GBS, so it's important to get tested for the infection during pregnancy. Learn more about GBS and pregnancy.
  • Pregnant women who get infected with genital herpes late in pregnancy have a high risk of infecting their fetus. The risk of infection is particularly high during delivery.1 Herpes infections in newborns are serious and potentially life-threatening. Infection with the herpes virus during pregnancy or at the time of delivery can lead to brain damage, blindness, and damage to other organs. Rarely, herpes infection during pregnancy can lead to serious complications in the mother, including severe liver damage and possibly death.
    • If a pregnant woman has had genital herpes in the past, there are medications that she can take to reduce the chance that she will have an outbreak, which also reduces the risk to her fetus.
    • If a woman has active herpes sores when she goes into labor, the infant can be delivered by cesarean section to reduce the chance that the infant will come in contact with the virus.5
  • If a woman is infected with hepatitis B virus (HBV) during pregnancy, the virus could infect her fetus. The likelihood of transmission depends on when during pregnancy the mother was infected. If the mother gets the infection later in her pregnancy, the risk that the virus will infect her fetus is quite high. If the infection occurs early in pregnancy, the risk of the virus infecting the fetus is much lower. For more information about Hep B during pregnancy, visit the Centers for Disease Control and Prevention (CDC) website. In infants, HBV can be serious and can lead to chronic liver disease or liver cancer later in life. In addition, infected newborns have a very high risk of becoming carriers of HBV and can spread the infection to others.
    • In some cases, if a woman is exposed to HBV during pregnancy, she may be treated with a special antibody to reduce the likelihood that she will get the infection.
    • All healthy infants should be vaccinated against HBV to give them lifelong protection.
    • Infants born to women with evidence of ongoing HBV infection (HBV surface antigen positive) should also receive hepatitis B hyperimmune globulin as soon as possible after birth.7
  • Hepatitis C virus (HCV). CDC offers more information about HCV.
  • HIV/AIDS. HIV can be passed from mother to infant during pregnancy before birth, at the time of delivery, or after birth during breastfeeding.6
  • Human papillomavirus (HPV). CDC offers more information about HPV, including vaccine recommendations.
  • Listeria or listeriosis is a serious infection usually caused by eating food  contaminated with a particular type of bacteria. Infection during pregnancy can lead to pregnancy loss, stillbirth, preterm birth, or life-threatening infection of the newborn. Listeriosis is most often associated with eating soft cheeses and raw milk, but recent outbreaks have been associated with fresh and frozen produce. Prevention recommendations include checking food labels to avoid eating unpasteurized cheese (made from raw milk) and other actions. Learn more about preventing listeria during pregnancy.
  • Lyme disease is the most common vector-borne disease in the United States. It is caused by strains of the bacteria Borrelia and is transmitted through the bite of certain species of ticks. The National Institute of Allergy and Infectious Disease has more information on lyme disease. In addition, the CDC provides a handout about pregnancy and lyme disease (PDF 1.2 MB)
  • Getting rubella (sometimes called German measles) during pregnancy can cause problems with the pregnancy as well as birth defects in the infant. Health care providers recommend that women get vaccinated against rubella before they get pregnant. Learn more about rubella and pregnancy.
  • Syphilis may pass from an infected mother to her fetus during pregnancy. The infection has been linked to preterm birth, stillbirth, and, in some cases, death shortly after birth. Untreated infants who survive tend to develop problems in many organs, including the brain, eyes, ears, heart, skin, teeth, and bones. All pregnant women should be screened for syphilis during their first prenatal visit. Women considered to be high risk should be screened again in the third trimester.1
  • Toxoplasmosis is a disease caused by a parasite that can be present in cat feces or used cat litter. Cats get the parasite from eating small animals or birds. In humans, the disease is usually mild, but if the parasite passes from a pregnant woman to the developing fetus, it can cause intellectual disabilities, blindness, or other problems. Women who are trying to become pregnant or are pregnant can take steps to prevent exposure to the parasite, such as having someone else clean or change the cat litter box and wearing rubber gloves to handle cat litter or while gardening.
  • Trichomoniasis. CDC offers more information about trichomoniasis.
  • Zika is caused by a virus spread mainly by the bite of a certain type of mosquito, but it is also spread through sexual contact. Although its symptoms are usually mild, Zika infection during pregnancy can cause pregnancy loss and other pregnancy complications, as well as birth defects and other problems for the infant.

CDC provides additional information on infections during pregnancy and sexually transmitted infections (STIs) and pregnancy.


  1. Centers for Disease Control and Prevention. (2016). STDs during pregnancy - CDC fact sheet (detailed). Retrieved January 17, 2017, from
  2. Rours, G. I., Duijts, L., Moll, H. A., Arends, L. R., de Groot, R., Jaddoe, V. W., et al. (2011). Chlamydia trachomatis infection during pregnancy associated with preterm delivery: A population-based prospective cohort study. European Journal of Epidemiology, 26(6), 493–502. Retrieved January 18, 2017, from
  3. Centers for Disease Control and Prevention. (2010). Cytomegalovirus (CMV) and congenital CMV infection. Retrieved December 28, 2015, from
  4. Krause, P. R., Bialek, S. R., Boppana, S. B., Griffiths, P. D., Laughlin, C. A., Ljungman, P., et al. (2013). Priorities for CMV vaccine development. Vaccine, 32(1), 4–10. Retrieved January 18, 2017, from
  5. Medline Plus. (2017). Genital Herpes. Retrieved January 18, 2017, from
  6. American College of Obstetricians and Gynecologists. (2012). HIV and pregnancy. Retrieved January 18, 2017, from external link
  7. The American College of Obstetricians and Gynecologists. (2019). Hepatitis B and hepatitis C in pregnancy. FAQ093. Retrieved December 30, 2020, from external link

What is labor?

Labor is the process by which the fetus and the placenta leave the uterus. Delivery can occur in two ways, vaginally (through the birth canal) or by a cesarean (surgical) delivery.

NICHD provides detailed information about these topics on its Labor and Delivery page.

What is a cesarean delivery?

A cesarean delivery is a surgical procedure in which a fetus is delivered through an incision in the mother’s abdomen and uterus.1

We offer more information about cesarean delivery in our Labor and Delivery topic.


  1. American College of Obstetricians and Gynecologists. (2015). Cesarean birth (C-section). FAQ006. Retrieved June 1, 2016, from external link (PDF 211 KB)
  2. Centers for Disease Control and Prevention. (2015). Births: Final data for 2014. National Vital Statistics Reports, 64(12). Retrieved June 1, 2016, from (PDF 2.95 MB)

What should I know about postpartum depression?

It’s not just postpartum, and it’s not just depression.

Many people are familiar with the phrase “postpartum depression,” or depression that occurs after the birth of a baby. But, we know now that it’s not just during the postpartum period, and it’s not just depression.

Women experience depression and anxiety, as well as other mental health conditions, during pregnancy and after the baby is born. These conditions can have significant effects on the health of the mother and her child.

The NICHD-led Moms’ Mental Health Matters initiative is designed to educate families and health care providers about who is at risk for depression and anxiety during and after pregnancy, the signs of these problems, and how to get help. The initiative also includes information for partners, family, and friends on ways to help.

Check out this video to learn more:

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The Maternal and Child Health Bureau, part of the Health Resources & Services Administration, offers the National Maternal Mental Health Hotline to help new mothers address the range of sometimes overwhelming emotions that can occur after their baby is born. The hotline provides free and confidential access to trained counselors and resources 24 hours a day, 7 days a week in English, Spanish, and more than 60 other languages. They can offer support and information related to before, during, and after pregnancy.

NICHD Pregnancy Research Goals

Improving the health of mothers during pregnancy and of their fetuses and children has been a primary goal of NICHD since its founding in 1962.

Various NICHD components conduct and support research to advance scientific evidence and develop new information and treatments to improve pregnancy outcomes for mothers and infants.

Some of the goals for NICHD related to pregnancy include studies to:

  • Understand the biology of pregnancy at a molecular level.
  • Help define an optimal pregnancy in terms of fetal, maternal, and paternal biology.
  • Advance knowledge and treatment of premature labor and preterm birth.
  • Evaluate environmental effects on the developing fetus.
  • Advance training for scientists.
  • Advance the development of technology, including the application of genomics, proteomics, and bioinformatics for pregnancy research.
  • Further research on the appropriate use of medication in pregnancy.

Pregnancy Research Activities and Advances

Many programs within NICHD's Division of Intramural Population Health Research (DIPHR)Division of Extramural Research (DER), and Division of Intramural Research (DIR) conduct and support research on pregnancy. Researchers investigate a wide variety of topics related to improving the health of mothers and their fetuses, from before conception to the weeks and months after birth.

In 2014, NICHD launched the Human Placenta Project (HPP) to address gaps in our understanding of this vital organ and develop tools to study the structure and function of the placenta in real time. Ultimately, research stemming from this initiative could improve pregnancy outcomes and the health of the child and mother throughout life. Each year, NICHD hosts a meeting to bring researchers from the placenta field and other fields together to discuss recent advances in the HPP.

Details about studies in the areas of pre-pregnancy care and prenatal care, pregnancy complications, and labor and delivery follow below.

Pre-Pregnancy and Prenatal Periods

NICHD seeks to increase awareness about the importance of the pre-pregnancy and prenatal periods for promoting a healthy pregnancy and the best outcomes for mothers and babies. Areas of investigation include:

  • Investigating ways to improve the health of women and their children before, during, and after pregnancy. Studies supported by the Pregnancy and Perinatology Branch (PPB), within NICHD's DER, focus on the development of healthy babies; finding better ways to diagnose, treat, and prevent disease in pregnant women and newborns; increasing infant survival by reducing the number of preterm births and other birth complications; and evaluating the safety and effectiveness of devices and instruments for better care of mothers and their babies.
  • Evaluating techniques to develop and refine prenatal screenings. The DER's Intellectual and Developmental Disabilities Branch supports studies leading to the development and refinement of screening methods for conditions leading to intellectual and developmental disabilities.
  • Investigating the role of nutrition during pregnancy. Researchers supported by NICHD and the National Institute of Neurological Disorders and Stroke have reported new findings related to the interaction between folic acid supplementation, an important tool in prenatal care, and genetic pathways that could inform strategies to optimize the prevention of neural tube defects (NTDs) such as spina bifida (PMID: 20843827).

    In addition, researchers supported by the Pediatric Growth and Nutrition Branch of the DER are studying iron-deficiency anemia in newborns to determine if identifying and treating iron deficiency in pregnancy can reduce this prevalent problem.
  • Establishing a national standard for fetal growth. NICHD's DIPHR led a study called "The National Standard for Normal Fetal Growth" to find a standard of infant growth in both singleton and multiple pregnancies. Among the study's important findings to date, investigators characterized fetal growth in 1,737 low-risk pregnancies and found significantly different estimated fetal weights for white, Hispanic, Asian, and black infants at 39 weeks (PMID: 26410205). These ethnic differences in infant growth could lead to unnecessary tests and procedures in healthy pregnant women.
  • Studying medication use during pregnancy. The Effects of Aspirin in Gestation and Reproduction (EAGeR) Study, supported by the DIPHR, is examining the therapeutic value of low-dose aspirin in prenatal care. The research will analyze the effects of low-dose aspirin in combination with the intake of folic acid, compared with folic acid alone, on the incidence of miscarriage and other outcomes.

    The DER's Obstetric and Pediatric Pharmacology and Therapeutics Branch promotes research to improve the safety and effectiveness of medications for pregnant women. Some current studies include the study of clonidine to treat high blood pressure and metformin for the treatment of diabetes. In addition, a recent study on the effectiveness of Diclectin for nausea and vomiting found that the medication resulted in dramatically improved symptoms compared to placebo.
  • Evaluating effects of drinking alcohol and smoking tobacco on pregnancy outcomes. The Pregnancy and Perinatology Branch in the NICHD DER is studying alcohol use during pregnancy and its effects on infants. Other studies through the Prenatal Alcohol and SIDS and Stillbirth (PASS) Network are investigating the impact of alcohol use and tobacco smoking, both before and during pregnancy, on risk for Sudden Infant Death Syndrome (SIDS).

Pregnancy Complications

  • Preterm birth. NICHD served as the scientific lead for the Surgeon General's Conference on Preventing Preterm Birth in 2008, which developed the national agenda and action plan aimed at preventing preterm birth. This topic is an active research focus for NICHD. The following current or recently completed research efforts are supported by the PPB:

    • Evaluating whether treatments with antenatal steroids between 34 and 36 weeks of pregnancy will decrease infants' need for oxygen support. This trial found that treatment with antenatal steroids significantly reduced respiratory complications in singleton pregnancies at risk for late preterm birth (PMID: 26842679).
    • Studying women who are pregnant for the first time, with the goal of identifying factors in women who may be at risk for complications, including pregnancy-induced hypertension, preterm delivery, and low-birth-weight infants, during their first pregnancy.

    Researchers participating in the Maternal-Fetal Medicine Units Network, also supported by the PPB, found that use of progesterone by women at risk for preterm birth due to a prior preterm birth reduces the chances of a subsequent preterm birth by one third

    NICHD research formed the basis of a recent change to the gestational period within a pregnancy that is recognized as "full term." According to the new designations external link:

    • Early term is defined as 37 weeks through 38 weeks and 6 days.
    • Full term is defined as 39 weeks through 40 weeks and 6 days.
    • Late term is 41 weeks through 41 weeks and 6 days.
    • Postterm is 42 weeks and beyond.

    NICHD research documented poorer health outcomes, including a 20% greater risk of breathing, feeding, and temperature problems, among babies born at 37 and 38 weeks gestation compared to those born at 39 weeks or later. These findings contributed to the designation changes (PMID: 23645117). The findings and the designation changes are the focus of NICHD's Know Your Terms initiative.

    In addition, the Perinatal and Obstetrical Research Affinity Group within DIR studies aspects of preterm birth and other pregnancy complications with the aim of improving outcomes and reducing short- and long-term effects.

  • Gestational diabetes. The DIPHR Epidemiology Branch is currently studying the increased risk of hypertension in women with gestational diabetes mellitus. In addition, researchers in this Branch recently reported that women with gestational diabetes may be able to prevent the development of type 2 diabetes by following a healthy diet after pregnancy. Read more about this finding. More recently, these investigators found that following a healthy diet after a pregnancy with gestational diabetes also reduces the risk of high blood pressure.

    Another study, called the Hyperglycemia and Adverse Pregnancy Outcomes (HAPO) Follow-up Study, is supported by the PNGB. This study will explore whether elevated blood sugar during pregnancy influences later levels of body fat in children and development of diabetes in mothers following birth.

    In March 2013, NICHD sponsored the NIH Consensus Development Conference: Diagnosing Gestational Diabetes Mellitus. The expert panel recommended following the existing two-step process for diagnosing GDM as opposed to adopting a one-step process.
  • Preeclampsia. Studies are ongoing to determine whether abnormal levels of certain substances in the blood can predict preeclampsia. NICHD researchers and others have found that women who develop preeclampsia have higher levels of a substance called soluble fms-like tyrosine kinase 1 (sFlt1) and lower levels of placental growth factor and vascular endothelial growth factor than women who do not develop preeclampsia. Read more about preeclampsia research supported by NICHD.
  • Infections. Researchers investigating the expression and regulation of a group of innate immune receptors, called Toll-like receptors (TLRs), discovered that TLRs may play a role in infection-associated pregnancy complications by regulating the infection-induced inflammatory responses at the maternal-fetal interface.
  • Pregnancy loss (miscarriage and stillbirth). Another recent study supported by the PPB found that women experiencing two stressful life events in the year before their delivery were 40% more likely to have a stillbirth than were women who reported no such events. Investigators in the PPB's Stillbirth Collaborative Research Network also recently found that women who have a stillbirth delivery are at greater long-term risk for depression.

Labor and Delivery

  • The Consortium on Safe Labor, within the DIPHR Epidemiology Branch, evaluated the appropriateness of relying on the Friedman curve, which has traditionally been used to plot hours of labor against cervical dilation in centimeters, to guide decision making. This study found that labor progresses more slowly than previously believed. Researchers are tracking trends in preterm deliveries, practices such as induced labor, and how environmental and other factors may influence fertility, pregnancy, and pregnancy outcomes over time. Results from this study suggest that labor today may last longer than it did for women 50 years ago.
  • In addition, the Maternal and Pediatric Infectious Disease Branch supports research examining mother-to-child transmission of HIV and how this can be prevented during pregnancy and labor. Current research includes:
    • Studies on the epidemiology of HIV infection and complications in pregnant women and the safety of using new medications during pregnancy
  • Current studies in the Developmental Biology and Congenital Anomalies Branch of the DER include immunobiology of the placenta and maternal-fetal interactions. The Branch also funds opportunities for and studies on understanding embryonic development and the origin and development of structural birth defects.
  • The Gynecologic Health and Disease Branch of the DER supports studies of interventions for identifying, preventing, and treating women at risk for obstetric fistula and other pelvic floor disorders. The National Center for Medical Rehabilitation Research funds the Center for Research on Women with Disabilities (CROWD), which completed the Final Report of the National Study of Women with Physical Disabilities in 1999. That report found that pregnant women with disabilities face significant challenges in finding health care providers who are knowledgeable about their disability to help them manage their pregnancy.

To achieve its goals for research on pregnancy and related disorders, NICHD supports a variety of programs, networks, and centers. A number of examples are included below.

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