The average duration of human pregnancy is about 9 months which is called the gestation period.
Vigorous contraction of the uterus at the end of pregnancy causes expulsion/delivery of the foetus.
This process of delivery of the foetus (childbirth) is called parturition.
The signals for childbirth originate from the fully matured foetus and placenta which induce mild uterine contraction called the foetal ejection reflex.