The Placenta and How It Develops

By the time the developing being is 2 weeks old, the placenta, or afterbirth, has begun to develop. The villi, embedded in the lining of the uterus, are forming primitive blood vessels, which tap into the woman’s blood supply. The function of the placenta is to supply the growing being with the oxygen and nutrients it needs from the woman’s blood system and to pass to the woman the waste products it does not need.

placentaThe fetal blood is always separate from the maternal blood. Substances are issued back and forth through a semipermeable membrane. At one time, the placenta was thought to act as a barrier to materials that might hurt the fetus. However, it is now known that almost everything that enters the woman’s body—including viruses, drugs, nicotine, and alcohol—is passed to the fetus.

The placenta continues to grow until about 2 months before delivery, when it reaches its maximum size. At that time, it is about the size and shape of a dinner plate, approximately 8 to 9 inches in diameter, but thicker and heavier, weighing 1 to 2 pounds at birth. The side that is attached to the uterine wall is darkened and has sections like circular puzzle pieces in it. (See Figure 2.10)

Scarlike areas of tissue may be apparent. These are areas of calcification, and they denote places that have degenerated. Women who smoke during pregnancy have more of these than do women who do not smoke. The side of the placenta that is next to the fetus is white and smooth, being covered by a membrane, the amniotic sac. (See Figure 2.11)

Nutrients coming from the woman pass to the blood vessels in the placenta. From the placenta, they move through the umbilical cord into the blood circulating within the fetus.

Inside the umbilical cord are three blood vessels—one large vein and two smaller arteries. Nutrients travel from the placenta to the fetus through the vein. Waste products return to the placenta through the arteries to be passed into the woman’s system. A jellylike substance called Wharton’s jelly surrounds the blood vessels and helps to protect them.

The umbilical cord begins to develop during the second week after conception and usually grows to about 18 inches in length, although it is possible for it to grow to anywhere from 12 inches to 40 inches. After the birth of the baby, the cord is clamped and cut. A stump remains, but dries up and falls off in 7 to 10 days.