The placenta makes intrauterine life feasible. This structure is essentially a mass of blood sinuses formed by the placental septa. Into the sinuses extend chorionic projections from the fetal portion of the placenta, each covered with an enormous number of microscopic villi containing blood capillaries. Maternal blood flows into and out of these sinuses by means of a well-channeled system of vessels derived from the uterine wall. Fetal blood is led into the villi through the two umbilical arteries and then led back through the umbilical vein. As the blood moves through the villi, nutrients are absorbed and waste products are removed.

This is largely effected through simple diffusion; that is, since the concentration of oxygen and nutrients is greater on the maternal side of the placental barrier, they diffuse from the mother to the offspring. By the same token, fetal wastes (carbon dioxide, urea, and the like) diffuse from the villi into the maternal blood, from which they are excreted by the kidneys.

Placental Hormones

Aside from serving as food source and purifier, the placenta also secretes hormones, without which pregnancy cannot continue. When fertilization occurs, the corpus luteum, which normally degenerates at the end of each menstrual cycle, is maintained by chorionic gonadotropin hormone secreted by the developing ovum. However, after about the fourth month of pregnancy, the chorionic gonadotropin concentration drops to low levels, and the corpus luteum ceases to be stimulated sufficiently to produce the necessary high levels of estrogens and progesterone. At this time the placenta takes over the job and pushes the concentration of these hormones to well over 50 times their peak value during nonpregnancy.

Estrogen and progesterone are especially vital during pregnancy. In brief, estrogens thicken the uterine musculature, greatly enhance the uterine blood supply, enlarge the breasts, and facilitate embryonic development. Progesterone relaxes the uterine musculature until the time of birth, aids the development of the endometrium, prevents ovulation, and enlarges the breast glands.

A placental estrogen of special diagnostic significanse is estriol. The maternal urinary concentrations of ertriol increase throughout pregnancy and decrease just before delivery. If estriol levels suddenly fall or do not rise rapidly (during pregnancy), a fetal difficulty should be expected