Pregnancy is typically counted as 40 weeks in length. This duration starts from the first day of the woman's last menstrual period (LMP) rather than from the actual date of conception, which usually occurs about two weeks later. The unborn baby actually spends about 38 weeks developing in the womb. Pregnancy is divided into three trimesters, each lasting about 12 to 14 weeks. Full-term pregnancies generally last between 37 and 42 weeks, with 40 weeks being the average length used for estimating due dates.