Labour can be somewhat coldly described as process by which the products of conception are expelled from the mother. Natural labour can be thought of as having three components: power, passages, and passengers.
- primary involves uterine contractions
- secondary involves voluntary muscles of the
- diaphragm and abdominal walls
- bony: pelvic bone (see "pelvimetry")
- soft: uterus, vagina, vulva, supporting tissues
- passengers: the fetus (see "presentation"), placenta and membranes