Ideal start of labor is the time when the fetus is completely mature. Prolonged pregnancy is a condition after that stage where harmful changes on fetus can occur.
Placenta in prolonged pregnancy may stop supplying fetus with enough oxygen, and this can end up with brain damage and even death.
The number of deaths due to prolonged pregnancy is almost twice as many as the number of newborn deaths after normal pregnancy.
If the obstetrician suspects on prolonged pregnancy, he will stimulate the contractions (see labor induction). Your baby will be monitored and, if in danger, your birth will be accelerated by forceps or caesarean section.