From the time of conception, your baby will grow and change inside your uterus until he or she is born. Each trimester has a unique set of milestones.
In the first trimester, your baby will grow from a fertilized egg into a moving fetus with eyes, ears, and working organs. In the second trimester, your baby’s features develop and you may be able to feel your baby move. In the third trimester, your baby will grow rapidly to get ready for birth.
Your doctor will calculate your estimated due date during your first prenatal care visit. It will be about 40 weeks from the first day of your last menstrual period. Even though your baby is not conceived until about two weeks after you start your period, week one of your pregnancy is considered to be the week of your menstrual period.
Your first trimester lasts until the end of week 12. The second trimester lasts from week 13 to the end of week 26. Your third trimester is from week 27 until birth.
Most babies are born between weeks 38 and 42.