Pregnancy is the time during which a woman carries one or more offspring until delivery. Childbirth usually occurs about 38 weeks after the conception, or about 40 weeks after the last menstrual period. During your first OB/GYN doctor visit, the doctor will normally tell you the estimated childbirth date, or the ""due date"".
The actual length of pregnancy depends on various factors. For example, the first pregnancy tends to last longer than subsequent pregnancies. Fewer than 10% of births occur on the due date; 50% of births occur within a week of the due date, and almost 90% occur within two weeks of the due date.
More Information: More about Pregnancy