Pregnancy: Condition Information

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
At 37 weeks, the fetus is considered full-term; these infants can usually survive without support if delivered. However, unless there is a medical issue, it is better to wait until at least 39 weeks to deliver. This gives the lungs, brain, and liver time to fully develop.⁶

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.cfm [top]
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 (PDF - 280 KB) [top]
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]
Centers for Disease Control and Prevention. (n.d.). Preterm birth. Retrieved August 6, 2012, from http://www.cdc.gov/reproductivehealth/maternalinfanthealth/PretermBirth.htm [top]
March of Dimes. (2011). Prematurity research. Retrieved August 6, 2012, from http://www.marchofdimes.com/research/prematurity-research.aspx [top]
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 [top]