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


  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.


  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.


  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 (pronounced vaj-in-NOH-sis) 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) (pronounced sahy-toh-meg-uh-loh-VAHY-ruhs) 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 (pronounced PAHR-voh-vahy-ruhs) 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 (pronounced li-steer-ee-OH-sis) 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 (pronounced tok-soh-plaz-MOH-sis) 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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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

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