The Fluid Surrounding the Fetus

The amniotic fluid in which the fetus floats serves many purposes. It prevents the walls of the uterus from cramping the fetus and allows it unhampered growth and movement. It surrounds the fetus with a fluid of constant temperature which is a marvelous insulator against cold and heat. Above all it acts as an excellent shock absorber. A blow on the mother’s abdomen merely jolts the fetus, and it floats away.

It is easy to appreciate a pregnant woman’s special concern if she falls violently, is struck a hard blow on the abdomen, or is badly shaken in an accident. Of course pregnancy does not lessen her chance for injury, but external trauma rarely harms the unborn child. If there is no vaginal bleeding within an hour after the accident, it is almost an infallible rule that no damage has resulted to the pregnancy. Very rarely an accident may initiate premature labor, usually by an associated rupture of the membranes. One may state unequivocally that accidents occurring during pregnancy never cause birthmarks, malformations, or (save in the rare cases in which they cause miscarriage) any other ill effects to the child itself.

During intrauterine existence the fetus swallows amniotic fluid, which is manufactured by the inner cells of the membrane, the amnion, which surrounds it, and re-excretes the fluid back into the sac as fetal urine. At term (the end of nine months) there is usually about a quart of amniotic fluid, though amounts up to eight gallons have been observed. When the amount of fluid exceeds two quarts the abnormal condition is called hydramnios.

The Navel Cord

The fetus is attached by the umbilical cord, which runs from its navel, to the inner surface of the placenta. Continuations of the blood vessels of the fetus run through the cord and then course through the placenta. The umbilical cord, a semi-transparent, jelly-like rope, averages twenty-two inches in length, although cords from a half-inch to fifty inches have been reported. Some are straight, others twisted, and somej in rare instances, are even knotted. The cord contains three blood vessels—two arteries, and one vein.

The Placenta and Its Function in Fetal Nutrition

The placenta is a complex organ, through which the fetus absorbs food and eliminates its waste products. The blood of the mother and the blood of the fetus come in close proximity in the substance of the placenta, and materials pass over from one blood system to the other. If, for example, the mother’s blood contains more sugar than that of the fetus, the excess diffuses over into the fetal blood until relative equality in the two is reached. In this way sugar that the mother eats is fed to her baby. On the other hand, the excess carbon dioxide of the fetal blood goes over to the mother’s blood and is exhaled by her lungs. In this way the mother breathes for her child. Other waste products of the fetus are likewise absorbed by the mother’s blood and voided by her kidneys. It is to be noted that not all chemicals, vitamins, and hormones are in exact equality in the two circulations. For example, the amount of Vitamin C in the fetal blood is several times that in the maternal blood. This delicate interchange between mother and fetus is further illustrated by the observation that cigarette-smoking by the mother temporarily increases the rate of the fetal heart. The maximum effect occurs from seven to twelve minutes after the cigarette is first lighted.

Most Drugs Cross the Placenta to the Baby

Almost all drugs taken by the mother cross, via the placenta, from her circulation to that of the fetus. The majority, such as aspirin, barbiturate sleeping capsules, the digitalis family of heart drugs, etc., neither benefit nor harm the baby. On the other hand, some drugs administered to the mother actually treat the fetus as well. A conspicuous example is penicillin for the cure of maternal syphilis. Another example of the possibility of treating the baby by treating the mother is the inhalation of oxygen. If from some complication of labor the baby’s heart rate becomes slow and irregular, demonstrating that there is insufficient oxygen in the fetal tissues, pure oxygen breathed by the mother through an anesthetic mask will raise the level of oxygen in her blood and similarly elevate it in the baby. Oxygen administered to the mother often normalizes a depressed, irregular fetal heart rate.

Occasionally drugs taken by a mother during pregnancy can be deleterious to the fetus. An outstanding example is that of morphine addiction. When the mother is an addict, the fetus becomes habituated to the drug in utero, and, if not given morphine after birth, the baby will show grave withdrawal effects, sometimes fatal. As a result, the newborn must be given morphine at regular intervals in gradually declining doses over the first several days of life.

The bloodstreams of the mother and fetus are ordinarily quite separate, and the interchange of materials is carried on through a multicelled partition. Some of the smaller molecules pass intact back and forth through the separating membrane; the larger ones, like the fats, have to be broken down on one side of the barrier, then pass through, and be reconstituted on the other side. This all sounds quite remarkable, and it is. To feed a rapidly growing organism, to keep it supplied with oxygen, and to excrete its waste products is a huge, complex task. The placenta, a relatively small organ, weighing one-fifth to one-sixth as much as the fetus, does this with unmatched efficiency.

Do the Bloods of Mother and Fetus Mix?

Before the end of the eighteenth century it was held that the blood cells of the mother intermingled freely with those of the fetus. However, at this time a famous English obstetrician injected a dye into the blood vessels of a woman who had died undelivered, and the dye did not appear in the blood vessel1; of her fetus. This was thought to prove the independent integrity of the two vascular svstems. and the scientific world believed that the mother and the fetus never interchanged blood cells. Less than two decades ago, when the disease erythroblastosis was first described, some doubt was thrown on this concept, for in order to cause erythroblastosis an immune response has to be produced in the mother to some factor present in the fetal red blood cells— a factor which she would ordinarily lack. Theoretically, this requires the passage of fetal red blood cells into the maternal circulation.

Undoubtedly this occurs. Thus far it has been proved only that the mother’s blood cells get into the fetal vascular system, but it seems quite safe to postulate that the reverse also must be true. Since one cannot inject an unborn, living human fetus without endangering it, it is imposs:ble to demonstrate the passage of fetal blood into the maternal circulation. What can safely be done is to inject the mother’s bloodstream with unique, certainly identifiable red blood cells, and then analvze the baby’s blood for them after it is born. Two types of experiments have been performed. In one, doctors transfused mothers at varying short intervals before birth with compatible red cells tagged with radioactive atoms and after delivery checked the baby with a Geiger counter. They got a positive response. In another series of experiments they transfused the mother before birth from donors whose bloods contained some rare, abnormal cells which could be searched for later in the infant. The bloods chosen had the property of sickling—the red blood cells became sickle shaped imparl of remaining round—when placed in an incubator at 37.5 degrees centigrade. The ‘sickle’ cells were found in the baby’s blood in more than 80 per cent of the experiments.

The pendulum of thought has now swung back to eighteenth-century opinion—not with a complete swing, however, for it is not felt that the blood of the mother and baby intermix without restraint; it is more likely that during pregnancy and labor there is often a small temporary leak of blood cells from one system into the other.