A normal pregnancy is considered to last anywhere from 37 to 42 weeks. This is calculated from the first day of the last menstrual period. More commonly though, labour usually starts around 39 to 41 weeks.
The question of how labour starts is not yet completely answered. But there are a few schools of thought, including:
- Hormones released by the baby?
- Hormones released by the mother and baby?
- The unique hand over process where the uterine space becomes smaller (with lessening amniotic fluid) and the baby recognising it is time?
- The placenta starting to function at less than its peak, and the baby getting the message that it is time?
- Hormonal feedback from the placenta?
- A number of the above factors, or something else we haven't discovered yet.
There is some research that now supports the theory that the baby's brain sends chemical messages to the mother's body when the baby is ready to be born. It is still unclear how this actually works, or what triggers the messages to be sent. When labour begins the woman releases the hormone oxytocin, which makes her uterus contract in a rhythmic pattern.
Updated November 2007
Bernal A. Overview of current research in parturition. Exp Physiology. 2001 March. Vol. 86(2), pp.213-22.
Stables D. and Rankin J. Physiology in Childbearing with Anatomy and Related Biosciences. 2004, Bailliere Tindall, Edinburgh.