Placental expulsion
Placental expulsion (also called afterbirth) occurs when the
The third stage of labor can be managed actively with several standard procedures, or it can be managed expectantly (also known as physiological management or passive management), the latter allowing the placenta to be expelled without medical assistance.
Although uncommon, in some cultures the placenta is kept and consumed by the mother over the weeks following the birth. This practice is termed placentophagy.
Physiology
Hormone induction of placental separation
As the fetal hypothalamus matures, activation of the HPA axis (hypothalamic-pituitary-adrenal axis) initiates labour through two hormonal mechanisms. The end pathway of both mechanisms leads to contractions in the myometrium, a mechanical cause of placental separation, which is due to the shear force and contractile & involution changes that occur within the uterus distorting the placentome.
Fetal ACTH
ACTH increases fetal cortisol, which acts by two mechanisms:
- Increases Prostaglandin F2α, which both abolishes the progesterone block, and lowers the oxytocin receptor threshold; and increases expression of relaxin, stretching the pelvic ligaments
- Increases expression of PTGS in the fetal trophoblast cells
PTGS in turn produces prostaglandin E2, which is a catalyst for pregnenolone to C-19 steroids, such as estrogen. Estrogen increases:
- Vaginal lubrication
- Softening of collagen fibre structures in the cervix, vaginal, and associated tissues
- Increases contraction associated proteins (i.e. connexins)
- Placental shedding by physiological inflammation, note pathological inflammation often leads to retention of membranes (i.e. placentitis)
Fetal oxytocin
As the HPA axis activates, the posterior pituitary of the fetus begins to increase production of oxytocin, which stimulates the maternal myometrium to contract.
Cellular changes of placental separation
In the seventh month of pregnancy the MHC-I complexes increase in the interplacentomal arcade reduces the bi- and tri-nucleate cells, a source of immune suppression in pregnancy. By the ninth month the endometrial lining has thinned (due to loss of trophoblast giant cells) which exposes the endometrium directly to the fetal trophoblast epithelium. With this exposure and the increase in maternal MHC-I, T-helper 1 (Th1) cells, and macrophages induce apoptosis of trophoblast cells and endometrial epithelial cells, facilitating placental release. Th1 cells attract an influx of phagocytic leukocytes into the placentome at separation, allowing further degration of the extracellular matrix.
Vascular changes of placental separation
After delivery, loss of fetal blood return to the placenta allows for shrinkage and collapse of the cotyledonary villi with subsequent fetal membrane separation.[1]
Active management
Methods of active management include
Umbilical cord clamping
Active management routinely involves clamping of the umbilical cord, often within seconds or minutes of birth.
Uterine contraction
Uterine contraction assists in delivering the placenta. Uterine contraction reduces the placental surface area, often forming a temporary hematoma at their former interface. Myometrial contractions can be induced with medication, usually oxytocin via intramuscular injection. The use of ergometrine, on the other hand, is associated with nausea or vomiting and hypertension.[2]
Cord traction
Controlled cord traction (CCT) consists of pulling on the umbilical cord while applying counter pressure to help deliver the placenta.[3] It may be uncomfortable for the mother. Its performance requires specific training. Premature cord traction can pull the placenta before it has naturally detached from the uterine wall, resulting in hemorrhage. Controlled cord traction requires the immediate clamping of the umbilical cord.
A
Manual placenta removal
Manual placenta removal is the evacuation of the placenta from the uterus by hand.
Efficacy of active management
A
Retained placenta
A
Non-humans
In most mammalian species, the mother bites through the cord and consumes the placenta, primarily for the benefit of prostaglandin on the uterus after birth. This is known as placentophagy. However, it has been observed in zoology that chimpanzees apply themselves to nurturing their offspring, and keep the fetus, cord, and placenta intact until the cord dries and detaches the next day.
The placenta exists in most mammals and some reptiles. It is probably polyphyletic, having arisen separately in evolution rather than being inherited from one distant common ancestor.
Studies on pigs indicate that the duration of placenta expulsion increases significantly with increased duration of
References
- ^ Attupuram, N. M; Kumaresan, A; Narayanan, K; Kumar, H. Molecular Reproduction and Development Apr/2016, Volume 83, Issue 4, pp. 287 - 297
- ^ |intentional=yes}}.)
- ^ PMID 25631379.
- S2CID 42420108.
- ^ BMJ summary of the Cochrane group metanalysis, at Postpartum Hemorrhage: prevention Archived 2008-10-11 at the Wayback Machineby David Chelmow.
- PMID 15159125.