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Pregnancy: Condition Information

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What is pregnancy?

Pregnancy is the term used to describe the period in which a woman carries a fetus inside of her. In most cases, the fetus grows in the uterus.

Pregnancy usually lasts about 40 weeks, or just over 9 months, as measured from the last menstrual period to childbirth. Pregnancy is divided into three trimesters. The major events in each trimester are described below.1

First Trimester (Week 1 to Week 12)

The events that lead to pregnancy begin with conception, in which the sperm penetrates the egg produced by an ovary. The zygote (fertilized egg) 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 formed from the egg and sperm. These cells form the fetus and the placenta. The placenta provides nutrients and oxygen to the fetus.2

Second Trimester (Week 13 to Week 28)

  • At 16 weeks, and sometimes as early as 12 weeks, a woman can typically find out the sex of her infant. Muscle tissue, bone, and skin have formed.
  • 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 heart problems.3

Third Trimester (Week 29 to Week 40)

  • 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 According to the March of Dimes, as many as 70% of preterm births occur between 34 and 36 weeks—these are late-preterm births.5
  • Infants born in the 37th and 38th weeks of pregnancy—previously considered full 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.
  • 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 the mother and baby are healthy, it is best to deliver at or after 39 weeks to give the infant’s lungs, brain, and liver time to fully develop.6
  • Infants born at 41 weeks through 41 weeks and 6 days are considered late term.4,5
  • Infants who are born at 42 weeks and beyond are considered post term.4,5

  1. Department of Health and Human Services. (2010). Stages of pregnancy. Retrieved August 6, 2012, from http://womenshealth.gov/pregnancy/you-are-pregnant/stages-of-pregnancy.html [top]
  2. American College of Obstetricians and Gynecologists. (2010). FAQs: How your baby grows during pregnancy. Retrieved July 31, 2012, from http://www.acog.org/~/media/For%20Patients/faq156.pdf?dmc=1&ts=20120731T1026504777 External Web Site Policy (PDF - 280 KB) [top]
  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. [top]
  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 http://jama.jamanetwork.com/article.aspx?articleID=1685467 External Web Site Policy [top]
  5. American College of Obstetricians and Gynecologists Committee on Obstetric Practice and Society for Maternal-Fetal Medicine. (2013). Committee Opinion No. 579. Definition of term pregnancy. Obstetrics & Gynecology, 122(5), 1139–1140. Retrieved November 11, 2013, from http://www.acog.org/Resources_And_Publications/Committee_Opinions/Committee_on_Obstetric_Practice/Definition_of_Term_Pregnancy External Web Site Policy [top]
  6. March of Dimes. (2011). Why at least 39 weeks is best for your baby. Retrieved July 31, 2012, from http://www.marchofdimes.com/pregnancy/getready_atleast39weeks.html External Web Site Policy [top]

Last Updated Date: 12/19/2013
Last Reviewed Date: 12/06/2013