The average gestation period in humans is 280 days or 40 weeks. The gestation period begins on the first day of the woman's last menstrual period. Full-term babies are born from 37 weeks to 42 weeks of the estimated date of birth.
The stages of human gestation are broken into three trimesters that are 3 months long each. The first trimester runs until the 12th week of pregnancy. The second trimester begins in week 13 and ends in week 28. The third and final trimester begins at week 29 and ends when the baby is born. Babies born before 37 weeks of gestation are considered to be premature. Irregular menstrual cycles can make it hard to determine the length of gestation, but ultrasound examinations can help pinpoint approximately how far along in pregnancy a woman is.