Pregnancy is the term used to describe the period in which a woman carries a fetus inside of her. 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

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. During this trimester, the organs of the fetus (baby) are formed.

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.

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.

Infants born in the 37th and 38th weeks of pregnancy—previously considered full term—are now considered “early 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.

Infants born at 41 weeks through 41 weeks and 6 days are considered late term.

Infants who are born at 42 weeks and beyond are considered post term.