Even in perfectly normal pregnancies, complications can arise when it comes time to deliver the baby. The timing of labor is actually very important. If the baby comes too early, it can suffer severe complications and medical issues, but if the pregnancy goes too long, the placenta can stop working properly, leading to birth injuryas well.
A pregnancy is considered full-term at 37 weeks. Any birth that takes place before 37 weeks gestation is considered preterm. Usually a baby born after 35 weeks suffers no medical problems, so labor is not stopped from 35 weeks on.
Sometimes labor begins on its own, either due to a medical problem the mother or baby may have, or for no reason whatsoever. Only about 10% of women deliver on their due date, although 90% of women deliver within 2 weeks of their due date. Early in the pregnancy, an ultrasound is done to try and determine the length of the pregnancy to date. Ultrasounds done in mid-pregnancy, or late pregnancy are less effective at determining the length of the pregnancy.