While the average pregnancy lasts about 38 weeks, and 40 weeks is considered as post-term, the longer the pregnancy continues after 40 weeks, the greater the risk of problems to the baby.
Most obstetricians will induce labor and deliver any pregnant patient when they reach 42 weeks gestational age. But because of the risk to the baby from a post-term pregnancy, after 40 weeks, the mother must be evaluated twice a week with non-stress testing of the fetal heart rate and ultrasound exams to determine the condition of the fetus.
The major risk to the baby comes from the fact that the placenta is designed to last 38-40 weeks and will provide adequate oxygen and blood supply to the fetus up to that time, but after 40 weeks the aging placenta is less able to supply sufficient oxygen to the baby. Thus, there is an increased risk of hypoxic brain injury to a baby in a post-term pregnancy.