Definition of Parturition
Parturition is the physiological process of childbirth, encompassing the coordinated events that lead to delivery of the fetus and placenta through labor and birth. 1
Core Components of Parturition
Parturition consists of five distinct but integrated physiological events that occur in sequence 2, 3:
- Fetal membrane rupture - breakdown of the amniotic sac 3
- Cervical effacement and dilatation - softening and opening of the cervix 3
- Myometrial contractility - coordinated uterine contractions that transition from irregular contractures to regular, forceful contractions 2
- Placental separation - detachment and expulsion of the placenta (third stage of labor) 3
- Uterine involution - return of the uterus to pre-pregnancy state 4, 3
Hormonal Mechanisms
The process is driven by a carefully orchestrated hormonal cascade 1:
- Progesterone withdrawal removes the dominant block to labor that maintains uterine quiescence throughout pregnancy 1
- Estrogen activation enhances uterotonic signals and prepares the uterus for labor 1
- Prostaglandin production reaches maximum levels when COX-2 enzyme expression peaks at delivery, serving as the final common mediator that stimulates parturition across all species 4, 3
- Oxytocin signaling increases through elevated oxytocin receptor (OTR) expression near term, promoting coordinated uterine contractions 4, 1
Molecular Changes at Parturition
Key contractile-associated proteins show dramatic changes 4:
- Connexin-43 (Cx43) expression increases to form gap junctions that coordinate myometrial contractions, then decreases rapidly to undetectable levels within 24 hours after delivery 4, 5
- Oxytocin receptors fall shortly after parturition and reach baseline values within 1-2 days 4, 5
- COX-2 expression is maximal at delivery, then returns to pre-pregnancy levels during involution 4
Clinical Context
Parturition represents a defining physiological moment that occurs 140 million times annually worldwide 6. The process involves both endocrine signals (hormonal changes) and mechanical signals (uterine distention), which work together to initiate and sustain labor 4. In humans and nonhuman primates, labor is superimposed upon pregnancy rather than representing a withdrawal from the pregnant state 2.
The immediate postpartum period involves rapid reversal of pregnancy-related changes, with many pregnancy-regulated genes returning to pre-pregnancy values within 1 day after delivery 4, 5.