An organ of life, the placenta nourishes the unborn baby. The placenta manufactures several hormones that support pregnancy, such as growth hormones, estrogen, progesterone and human chorionic gonadotropin (hCG), which triggers a positive pregnancy test. The placenta can also exchange gases, filter wastes, release glycogen and regulate glucose for the fetus, functions that will be taken over eventually by the newborn lungs, kidneys, liver and pancreas.
Early Development
At the end of the second week of pregnancy, primitive chorionic villi of the placenta appear. During the implantation process, enzymes open capillaries in the uterine wall and the villi grow outward from the embryonic sac into maternal tissue. Both maternal and embryonic tissues expand until there are sufficient surfaces to provide gas and nutrient exchange.
Roughly 70 percent of multiple pregnancies, according to Anne Frye, author of "Holistic Midwifery," originate from two fertilized eggs. The babies will have separate placentas, although they may fuse together at the edges. Identical twins originate from one fertilized egg. Depending on when the split takes place, they may have separate placentas or share a common one.
Size
By the 16th week of gestation, maternal arteries in the uterine wall have enlarged and softened, becoming uteroplacental vessels. The fetal part of the placenta, with the bag of waters, separates the baby from the mother's circulation. Only the three vessels of the umbilical cord connect placental circulation with fetal circulation.
The placenta is fully developed by the 18th week of pregnancy. According to Frye, it covers between 15 and 30 percent of the lining of the uterus. A term placenta weighs roughly one-sixth the weight of the newborn.
Function
Fetal blood is pumped through the umbilical arteries to the extensive arteriocapillary-venous system within the villi of the placenta, where maternal and fetal blood meet but do not mix. Nutrient-rich, oxygenated blood returns to the fetus through the umbilical vein. Frye asserts that the physical health of the fetus and placenta is dependent upon the adequate bathing of the villi with maternal blood.
Significance
As pregnancy advances, the the placental membrane becomes thinner. Oxygen, water, carbohydrates, amino acids, lipids, vitamins, electrolytes and some antibodies cross the placenta easily. However, most medications, drugs, chemicals, carbon monoxide, nicotine, alcohol, some bacteria and many viruses may also enter fetal circulation.
Delivery
Delivery of the placenta is called the third stage of labor. An average of 30 minutes after the baby is born, those in attendance will note signs of placental separation. The umbilical cord lengthens and a characteristic gush of blood signals that the placenta is ready to deliver. The mother may feel like pushing. According to Elizabeth Davis, author of "Heart and Hands," the placenta may deliver with either fetal or maternal side presenting, depending on whether separation started at the center or the edges.
References
- Hollistic Midwifery; Anne Frye; 1998
- Heart and Hands; Elizabeth Davis; 2004
