To sum up the normal pregnancy, we find that in the course of ten lunar months, following the fertilization of an ovum, the uterus grows from a small, flattened, pelvic organ, three inches in length, to a large, globular, muscular sac, constituting an abdominal tumor about fifteen inches long; it increases its weight thirty-two times, that is from two ounces to two pounds, while the capacity of the uterine cavity is multiplied five hundred times. Within the cavity is a child weighing about seven and a quarter pounds, surrounded by a quart or so of amniotic fluid. This fluid is contained in the sac composed of the fetal membranes, the amnion and chorion, which are excessively developed at one point into the placenta. The placenta, in turn, is attached to the child by means of the umbilical cord. The total weight of the uterus and its contents at term is usually about fifteen pounds.
Quite as mysterious and inexplicable as the development of these complex structures from one tiny cell is the fact that when the new human being is ready to begin life as a separate entity, further changes occur within the mother’s body which produce uterine contractions of such a character as to entirely empty the uterus of its contents.
CHAPTER IV
GROWTH AND PHYSIOLOGY OF THE FETUS
Although the fetus at term is in many respects simply a diminutive, immature man, or woman, its anatomy and physiology present certain characteristics which have adapted it to a protected existence in a sac of fluid. Some of the fetal structures and functions become increasingly active after birth, while others subside and disappear.
We have seen that after the first month of pregnancy the placenta serves the fetus as a combined respiratory and digestive apparatus, not alone in supplying the oxygen and nourishment requisite for life and growth, but also in excreting the broken-down products of fetal life. It apparently acts somewhat as a liver, too, in performing something akin to a glycogenic function.
Obviously, then, the fetus must possess a circulatory mechanism which is peculiar to itself alone, and not found in the independently existing human body, in which the lungs and alimentary tract are functioning as intended. This mechanism is provided by means of certain structures which exist in the fetal circulatory system and which automatically disappear shortly after birth. The nurse must be aware of these anatomical changes that take place, in addition to growth, if she is to have an intelligent grasp of her tiny patient’s possible needs.
The structures which change or disappear after birth are the foramen ovale, a direct opening between the right and left auricles, and four blood vessels: the ductus arteriosus, ductus venosus and the two hypogastric arteries. An understanding of the functions of these vessels involves an understanding of the course followed by the fetal blood currents, as indicated in Fig. [28], page [85].
We see that there are three vessels within the umbilical cord: the umbilical vein and two arteries. In spite of its name, the vein conveys arterial blood from the placenta to the fetus. After piercing the baby’s abdominal wall, it divides into two vessels; the larger one, called the ductus venosus, empties into the inferior or ascending vena cava, while the smaller branch joins the portal vein, which enters the liver. The relatively large amount of arterial blood sent directly to the liver may in part account for the large size of this organ in the fetus. Upon its emergence from the liver, this blood stream flows into the inferior vena cava.
Fig. 28.—Diagram showing course of fetal circulation through hypogastric arteries, ductus venosus, ductus arteriosus and the foramen ovale. (From The American Text Book on Obstetrics.)