The
third stage of labour involves the birth of the
placenta. Within 5 to 20 minutes of the baby being born, the woman's uterus contracts again and in doing so reduces the uterus' size. This action shears the placenta off the wall of the uterus so that it can come down and out the birth canal.
Image 3-11 shows the uterus shrinking and the placenta separating.
Updated November 2007
Information sources
Stables D. and Rankin J. Physiology in Childbearing with Anatomy and Related Biosciences. 2004, Bailliere Tindall, Edinburgh.