How Many Weeks Does a Pregnancy Last?

If you divide 40 weeks by 4, you end up with 10 months. The reason for the discrepancy is that some months are shorter than others. Despite this, the average pregnancy does last for about 9 calendar months.

The 40 weeks of pregnancy are counted from the first day of the mother's last menstrual period, which happens before she is actually pregnant. Since a woman's menstrual cycle is an essential part of pregnancy, it is considered the true start of a pregnancy. A woman typically discovers that she is expecting during the 4th or 5th week of pregnancy, or 2-3 weeks after a baby is conceived.

While 40 weeks is a full term pregnancy, many OB/GYNs consider a pregnancy full term and safe to deliver between 38-40 weeks of gestation. The risk of complications during and after delivery is much lower after a pregnancy has reached the 38th week. By this point, a baby should be fully developed and able to survive outside the womb.

A baby that is born before 37 weeks of pregnancy is considered a preemie. The farther along the mother is into her pregnancy, the better the baby's chances of being born healthy and with no long-term complications. Due to pre-term labor or health concerns for the mother or baby, an OB/GYN may opt to deliver the baby early. In some cases the doctor may believe that an early delivery is best for both mother and baby. In other cases an early delivery is unavoidable despite the best efforts of medical staff. With the advancement of modern medicine, a preemie has a good chance of survival after the 28th week of pregnancy, but many babies born too early develop lifelong medical conditions.

A micro preemie is a baby born before 26 weeks of pregnancy or weighing 1 3/4 pounds at birth. While many micro preemies do survive, a variety of complications can arise, many of which develop into lifelong conditions. Doctors do everything they can to prevent a baby from being born early to increase the chances for a positive outcome for the mother and baby.

A baby is considered overdue when he is born after the 40th week of pregnancy. Some doctors will allow an expectant mother to go up to two weeks overdue, or through the 42nd week of pregnancy. Other doctors may begin discussing induction when a mother reaches the 40th week of pregnancy. The decision is based on many factors, such as a mother's medical history, any complications with the current pregnancy and a doctor's professional opinion.