Expansion
Cervical dilatation, delivery of the fetus, then delivery of the placenta
Expansion
Stages
- First: onset of regular contractions to full dilatation (10 cm). Latent phase to 4 cm, then active phase
- Second: full dilatation to delivery of the baby
- Third: delivery of the placenta, normally within 30 minutes
Hormonal changes initiating labour
- Falling progesterone action relative to oestrogen, increasing myometrial excitability
- Increased oxytocin receptor density on the myometrium, which matters more than the oxytocin level itself
- Prostaglandins E2 and F2 alpha causing cervical ripening and contraction
- Fetal cortisol contributing to the hormonal switch
- Increased gap junctions between myometrial cells, allowing coordinated contraction
Ferguson reflex: cervical and vaginal stretch stimulates oxytocin release, which increases contraction, which increases stretch. This positive feedback continues until delivery breaks the loop.
Third stage haemostasis depends on myometrial contraction compressing the spiral arteries, the so-called living ligatures, rather than on clotting. This is why uterine atony causes torrential haemorrhage and why oxytocics are given routinely.