There are four major phases of parturition, including quiescence (0), activation (1), stimulation (2), and involution (3). Quiescence in late pregnancy is characterized by uterotonic inhibitors and progesterone being quiescence. Activation (5% of pregnancy duration) includes the decrease in uterotopic inhibitors and estrogen increase. During the stimulation phase (0.2%), uterotonins contribute to labor progression and making contractions stronger/longer (Blackburn, 2013). Involution is the final stage when the uterus is transformed from pregnant to its former (non-pregnant) state.
Each phase has a number of inhibitors that define the quality and duration of labor:
- Phase 0: progesterone, prostacyclin, relaxin, and nitric oxide (Anderson & Avery, 2019);
- Phase 1: estrogen;
- Phase 2: oxytocin and prostaglandins;
- Phase 3: oxytocin and thrombin.