The Placenta

Dive Deeper
Human placenta shown a few minutes after birth. The side shown faces the baby with the umbilical cord top right. The unseen side connects to the uterine wall. The white fringe surrounding the bottom is the remnants of the amniotic sac. (Image Credit: Jeremy Kemp, 4/9/05, Public Domain)

The placenta and umbilical cord form as maternal and embryonic tissues interact.1 During implantation, the outer cells of the embryo, called trophoblast cells, invade the uterine wall. These trophoblasts almost immediately rupture blood vessels in the uterine wall and are bathed in maternal blood. Oxygen and nutrients start diffusing from the mother to the embryo by passive diffusion from this point on. The maturing blood circulation in the placenta is not functional until about 3 ½ weeks after the sperm and egg unite at conception.2

The placenta becomes the lifeline for the developing embryo. Blood vessels from the embryo interlace within the mother’s blood in the placenta.3 Oxygen and nutrients flow to the unborn child, while carbon dioxide and other wastes pass back to the mother through the placental circulation. The placenta produces the hormone progesterone, required for maintaining the pregnancy. Furthermore, the placenta serves as a barrier and protects the baby from many types of bacterial and viral infections.4

Male and female fetuses have small differences in placental hormone production and function.5 The placenta also maintains the fetal temperature above the mother’s temperature, between 100.7 and 102.5 degrees Fahrenheit.6 No modern technology can surpass the placenta’s life-support capabilities.

Sperm-egg fusion