Unit 9-Reproductive Processes

Chapter 28

Pregnancy, Delivery, and Lactation

1. Length of Pregnancy:

     There is fairly widespread confusion about the dating of pregnancy. Pregnancy is ordinarily timed from the first day of the last menstrual period. Thus, a woman fertilized on the 14th day would be considered "one month pregnant" 16 days later. A woman who knows exactly when conception occurred may think of the pregnancy as being, say, six weeks old, and may be surprised when she is told that she has a two month pregnancy.

     The normal time from fertilization to delivery is about 265 days. When the conventional method of timing is used, delivery can be expected 270 days after the beginning of the last menstrual period, though of course this figure is a rough approximation. Most women deliver between 275 and 295 days after the beginning of pregnancy, reckoned in the conventional way.

2. Fertilization, Early Development, and Implantation:

     It was noted in Chapter 26 that the ovum was set free when its follicle ruptured. Usually it is picked up by the flared ends of the Fallopian tube, and on rare occassions it passes into the abdominal cavity.

     Fertilization of the ovum is believed to occur in the Fallopian tube. It is doubtful that the ovum attracts sperm cells; more likely, there are so many of them (250,000,000 in an average ejaculation) that some will reach the Fallopian tube by chance. Sperm are motile, moving at 2mm / min, but, lacking direction, their velocity over any great distance is likely to be very small.

     It has been suggested that a single sperm cell (shown in Figure 376) cannot break the ovarian barrier (Figure 377). The ovum is surrounded by cells derived from the ovary. These are held together by a cement substance which is broken down by hyaluronidase, present in sperm cells. Probably several sperm cells contribute the hyaluronidase used in the dissolution of this cement, but only one sperm cell is required for fertilization.

     The fertilized ovum, beginning its division, continues on its way through the tube; its progress is quite slow. It enters the uterine cavity 3-4 days later. The endometrium at this time is under the influence of the corpus luteum, and its glands are highly developed and actively secreting. Implantation of the fertilized ovum occurs about 5-6 days after it has entered the uterine cavity, 7 to 10 days after ovulation. The growth and development of the ovum before implantation are supported by materials stored in the ovum itself, the cells which cover it, and perhaps the fluids of the uterine cavity.

     By the 8th day after fertilization, the zygote, or fertilized egg, has divided many times and differentiated to form a fairly complex structure (Figure 378). Its outside is covered with a layer of cells (trophoblasts) which are highly invasive, secreting enzymes which can digest the materials of the endometrium. Inside, there are two cavities, both fluid filled, separated from each other by the germ disc. The germ disc contains all the cells which will develop into the embryo. One of the cavities is rather like the yolk of an egg and is called the yolk sac. The other cavity, which is at first quite small, will later enlarge and cover the whole embryo. It is called the amniotic cavity.

3. Formation of the Placenta:

     Contact between the trophoblast and the endometrium results in a localized endometrial breakdown. The embryo implants at the site of contact, and the endometrium becomes reconstituted over it, so that the embryo comes to be deeply imbedded in the endometrium. It should be noted here that trophoblastic activity is greatest on the side of the embryo which is most deeply implanted.

     The endometrium responds to trophoblastic invasion by cellular growth, decidual reaction, which thickens the endometrium and resists invasion. The invasion of the trophoblast nevertheless continues until the walls of uterine blood vessels are eroded. Blood now circulates from the uterine artery to the uterine veins through lakes of blood which the trophoblast has formed, and of which it is a border.

     The trophoblast now begins to send out finger-like projections into these blood lakes. Within these projections are blood vessels which serve the embryo, reaching it through the body stalk. The trophoblast-blood vessel combination is called the chorion, while the projections are called the chorionic villa (Figure 379). Note that maternal and fetal blood are separate from each other as long as the chorionic villi are intact.

     As development proceeds, the cborionic villi become more and more branched, the endometrial layer which forms in reaction to trophoblastic invasion becomes thicker, and the membrane lining the amniotic cavity, now overlying the entire embryo, fuses with the chorionic villi at their base. The whole structure, called the placenta, makes a bridge between the circulation of the embryo, called the fetus 18 weeks after conception, and that of the mother. Yet it is a bridge not ordinarily crossed by the blood itself. Only small molecules are transferred from one circulation to the other. Larger molecules, those of molecular weight more than 1000, do not cross the placental barrier. The structure of the placenta and its relationships to the circulation are shown in Figure 380.

4. Later Development of the Fetus:

     The course of embryonic development is very regular. Most of the important structures are laid out in the first 8 weeks, and most defects in the fetus usually result from faulty development in this period. At this time, the fetus is about 25 to 30 mm long and is recognizably human. The sex cannot be distinguished easily at this time, but by the tenth week it is usually possible to do so.

     From the fourth to the ninth month, there is minor differentiation and development, but there is continuous increase in size and weight. The student interested in the details of development will find these thoroughly treated in most textbooks of embryology.

5. Hormones Produced during Pregnancy:

     Implantation occurs just a little before menstruation would have occurred. Obviously, the embryo must act to maintain the endometrium in which it will live, and on which it is dependent.

     It will he recalled that the maintenance of the endometrium depends on continued activity of the corpus luteum, yet that very activity results in the production of hormones that "turn off" the pituitary. The function of maintaining the corpus luteum is, however, taken over almost as soon as implantation occurs, by a secretion of the trophoblastic cells, called chorionic gonadotrophin. This hormone maintains and stimulates the corpus luteurn which continues to produce both estrogens and progesterone throughout pregnancy, though it begins to degenerate in the fifth month. The placenta cells, however, have acquired the ability to produce estrogenic hormones and progesterone. At three months, they secrete enough of these hormones to maintain pregnancy, even if the ovary is removed. Thus, normal endometrial losses do not occur during pregnancy.

     The hormones of the corpus luteum have a further important effect in early pregnancy. The uterine muscle responds to foreign bodies by contraction and expulsion of the body. This contractility is enhanced by estrogenic hormones; the longer these are present, the more responsive is the uterus. On the other hand, progesterone masks the estrogen effect completely. The implanted embryo, a foreign body, would invite uterine expulsion were it not for the inhibitory effect of progesterone.

     Continued secretion of estrogens throughout pregnancy continuously enhances uterine contractility, but the uterus is always kept quiescent by the simultaneously secreted progesterone. In sum, the uterus gains responsiveness during pregnancy due to the estrogens, but it does not respond due to progesterone.

     At the same time, the continued secretion of estrogens and progesterone begins to produce marked development of the breasts, which may be so great as to cause discomfort. The development is a true growth. There is no secretion of milk during pregnancy.

     It is probable that the hormones of pregnancy have other effects. An aldosteronelike effect may be involved in the retention of sodium: blood and extracellular fluid volume may increase through this action. There is usually a weight gain in pregnant women, greater than the weight increase of the baby, uterus, and uterine contents, and extracellular fluid. This weight gain, normally 18 to 24 pounds, may be under endocrine control, but this has not been proved.

6. Diagnosis of Pregnancy:

     A presumptive diagnosis of pregnancy can be made when the following are noted in a woman who has had sexual relations with a man. These are missing a menstrual period, tenderness of the breasts, diminished bladder capacity, and "morning sickness". The first two of these symptoms are associated with implantation and secretion of chorionic gonadotrophin with the consequent hyper-secretion of estrogens and progesterone.

     Reduced bladder capacity results from the enlargement of the uterus, which overrides the bladder roof. There is no satisfactory explanation for morning sickness; it has been speculated that trophoblastic invasion, destroying parts of the uterus, is at fault. In addition to the above, there is usually a softening of the uterine wall at the site of implantation, and an unexplained softening of the portion of the uterine just above the cervix. This can be detected as early as the sixth week by a skillful physician performing a pelvic examination.

     The diagnosis of pregnancy is considered probable, but not certain, when, in addition to the above, the uterus is felt to be enlarged, when a sound of rushing blood can be heard over the uterus, and when an enlarged uterus contracts in response to pelvic examination. These signs occur quite late in pregnancy. The sound of rushing blood occurs after the 16th week, and uterine contractions occur after the 28th week.

     The positive diagnosis of pregnancy is made when a fetal heart beat can be heard (17 weeks), when fetal movements can be recognized by the physician (18 weeks), or when a fetal skeleton can be seen in the X-ray (12 weeks). The absolutely certain diagnosis of pregnancy is made when a physician notices a baby which was not there before, unless somebody is playing tricks on him, which is easier than most physicians admit. Of course, once the absolutely certain diagnosis of pregnancy is made, the woman is no longer pregnant, unless she is going to have twins, triplets, or quadruplets, a possibility usually overlooked by the busy obstetrician.

     Most laboratory tests for pregnancy are based on the fact that the trophoblastic secretion, chorionic gonadotrophin, appears in pregnant urine. This hormone is a protein, molecular weight 30,000, and has follicle stimulating as well as luteinizing effects in other species. Pregnant urine injected into isolated rabbits leads to ovulation and corpus luteum formation, shown in the Friedman test. In frogs and toads, sperm or egg cell production may be induced.

     This test becomes positive rather quickly: a couple of days after implantation or about two weeks after fertilization. It can, however, give falsely positive or falsely negative results. False positives occur in the presence of a disease of the placenta, in which the fetus is dead, though the placenta survives. False negatives are observed when the urine samples are taken too early after fertilization. For example, a woman with a 28 day cycle may ovulate and conceive on the 21st day. Implantation may occur on the 28th day, but measurable amounts of the gonadotrophin may not appear in the urine for another five days. When there is presumptive evidence of pregnancy and a negative laboratory test, the test should be repeated.

7. The Onset of Labor:

     It was noted in Part 5 of this chapter that the hormones of the placenta acted on the uterine muscle in different ways. Estrogens sensitize the uterus to stimulation, while progesterone prevents it from responding. It is possible, but not proved, that the event which initiates labor is the loss of placental ability to produce both these hormones. Falling levels of progesterone unmask the estrogen-induced hypersensitivity of the uterus, and any stimulus may now initiate contraction. There are several such: the posterior lobe of the pituitary secretes a polypeptide hormone, oxytocin, which can by itself cause uterine contraction. The baby can stimulate the muscle simply by movement, perhaps just by being there (it will be recalled that hollow muscular organs depolarize and contract when foreign bodies are present in their cavity).

     The normal position of the baby at term is head down (Figure 381). Any contraction of the fundus tends to force open the internal end of the cervix, the head acting as a broad wedge. Such cervical stretching may give rise to sensory impulses which cause oxytocin release, or possibly depolarization propagated through the myometrium to the fundus.

8. Labor:

     Whatever mechanism begins and sustains it, labor ordinarily follows a fairly regular course. It is divided into three phases: the first is from the onset of contractions to opening of the entire cervix; the second from the end of the first to the delivery of the baby; and the third phase begins at the end of the second and ends with the delivery of the placenta.

First Phase: Uterine contractions, also called labor pains, occur with increasing frequency and amplitude. With each pain, the cervix is opened a little more. The pains are, at first, spaced at 30 minute intervals, at the end they may occur every two minutes and last for one minute. The gradual opening of the cervix, called effacement, is diagrammed in Figure 382. This phase of labor may last as little as an hour or as long as 16 hours, but when it is longer than 16 hours, there are usually complications.

     Second Phase: When the cervix is effaced, the infant's head enters the vaginal canal. Voluntary "bearing down" may hasten this phase. It should, however, be remembered that the head of the infant is not too different in size from that of the pelvic outlet, and will fit into it only through turning, both being elliptical in shape. The vaginal and vulvar tissues are very distensible, but they must be distended slowly or they will tear. This phase is ordinarily very painful and lasts from 1 / 2 to 1 hour.

     When, as is usually the case, the back of the head is first to enter the canal, a series of bendings and rotations of the neck is required for passage through the pelvic floor, but these will not be discussed here.

     The baby's head finally presents at the vaginal orifice. To avoid an irregular tear, it is nowadays almost routine to make a clean cut (Figure 383). This is called an epesiotomy and can be repaired without significant scarring.

     Once the head is delivered, the rest of the baby follows in a few moments. The uterus, suddenly emptied, stops contracting for a while, and the placenta remains.

     The third stage of labor begins as the uterus becomes gradually smaller through gradual contraction. The placenta becomes geometrically unsuited (Figure 384) to the diminishing site of its attachment and begins to separate. It should always be remembered that the placenta acts as a stopper for the uterine blood lake. Its removal leaves an open wound in the endometrium and is very hazardous. Though the main business of labor is over by the end of the second stage, most of the complications result from mismanagement of the third stage than in the other two together.

     If all goes well, the uterus contracts further after placental separation and stays contracted. Labor is over. Some problems of the third stage will be discussed in Part 12 of this chapter.

9. Changes at Birth:

     The infant before birth was kept warm and surrounded by fluid which protected him from gravitational stress. He did not (in fact could not) breathe, and was, in short, almost totally deprived of stimuli.

     All this changes at birth. The infant, no longer surrounded by warm water, is stimulated to respond. Short of oxygen and high in carbon dioxide, the placenta no longer adequate to exchange these gases, his inspiratory center is stimulated. He inspires deeply, thereby changing his whole circulatory pattern. These changes will be described in Part 12 of this chapter.

10. Involution of the Uterus:

     The empty uterus is still much larger than the normal organ. In the course of the next six weeks, its normal size is restored. The cause of uterine involution is not known.

11. Lactation:

     During pregnancy, the breasts show considerable growth under the influence of estrogens and progesterone, but they do not secrete milk. Milk production results when the sex hormone-prepared breasts are exposed to lactogenic hormone from the anterior lobe of the pituitary. The secretion of prolactin appears to be inhibited by progesterone. When the latter disappears, with the placenta, prolactin is secreted and milk is produced.

     One of the silliest fads of this century was based on the assumption that this was not so. This was Dianetics--the Science of Mind--the creation of a writer of science fiction stories named Hubbard. (Hubbard, who has since become even sillier, had an immense following among people previously thought to be intelligent. It was his opinon, expressed in detail in a book called Dianetics--which was written in three weeks and looks it, that the unborn infant found life in the uterus a perfect hell, that it heard, and misunderstood, conversations and monologues of unprintable vulgarity, which he nevertheless printed, and that its personality was warped by these stimuli, as well as by knitting needles, used by most mothers to produce abortions, until it was relieved by talking to Hubbard or one of his disciples.

     The production of milk is only a first step. After production, it must be brought to the nipples. This process is a reflex whose efferent limb is endocrine. Suckling at the nipple stimulates hypothalamic centers which act to bring the milk to the nipple by releasing the hormone oxytocin. The continued production and release of milk depend on regular stimulation of the nipples by suckling. Prolactin and oxytocin are both produced through this kind of stimulation. The continued production of milk involves a number of hormones besides. Adrenal glucocorticoids seem to be necessary for the production of milk sugar, which is somewhat different from ordinary sugar; estrogens maintain the mammary glands and thyroid and growth hormone seem to play a role. The extent to which the whole organism is mobilized for milk production may be indicated by the fact that the caloric requirement of the lactating woman is 1 1/2 times that of the normal.

     Milk production can be stopped quickly by stopping breast feeding. Within two to three days, milk production is arrested. There is some discomfort, but it is easily controlled by pain relieving drugs.

12. Disorders of Pregnancy, Delivery, and Lactation:

     On the average, pregnancy lasts 270 days from fertilization to delivery. However, the system of reckoning from the first day of the last menstrual period makes the duration of pregnancy 284 days, the range being from 275 to 295 days. Babies born outside these limits are said to be premature or postmature. Premature babies tend to be small and underdeveloped: their temperature regulation is poor. These babies usually survive if the pregnancy is more than 210 days, but they demand very good nursing care. Some of them are put in oxygen tents. This procedure has been found to be rather unsafe, because infants exposed to excess oxygen may develop an opaque thickening of the back of the lens capsule, retrolentol fibroplasia, and are blinded. Others develop a thickening of the alveolar membranes, hyaline membrane disease, and may die of respiratory failure.

     Post-mature infants may be so large that normal delivery is difficult or even impossible. Some of these infants may require delivery by Caesarean section (see below). The ovum, fertilized in the Fallopian tube, normally progresses into the fundus of the uterus, implanting near its top or high at its side. There are, however, many cases where the ovum is fertilized outside the genital tract. It may implant and grow in the outside wall, of, for example, the large intestine. In other cases the fertilized ovum implants in the Fallopian tube. Any such pregnancy is called ectopic. The fertilized ovum covered by highly invasive trophoblast can invade and destroy most normal tissues, the uterine body is protected by its decidual reaction, which is protective. Other tissues, which lack the ability to react to trophoblastic invasion may simply dissolve before the advancing trophoblast. Blood vessel walls may be involved and one of the common complications of tubal pregnancy, an ectopic pregnancy in the Fallopian tube, is internal hemorrhage. These must be treated as medical emergencies; they should be suspected when after a missed menstrual period, there is cramping, one-sided abdominal pain, and usually, but not always, with some vaginal bleeding. The signs of hemorrhage and shock may appear (Chapter 16). Surgical treatment is mandatory.

     Implantation occurring in the lower parts of the uterus may have very grave consequences. The placenta may completely cover the cervix so that it will separate from the uterus in the first stage of labor, leading to uncontrollable maternal hemorrahage. Caesarean section is required in such pregnancies. Any low implantation site may result in placental separation during labor; some can be managed conservatively--that is without surgery--but surgery should always be considered. The condition is called placenta previa (Figures 385 and 386).

     Trophoblastic tissue sometimes survives after fetal death. In some cases it develops in an irregular patterns, invading the uterus at multiple sites. Upon removal the tissue looks like a bunch of grapes, because of the swelling of the chorionic villi.

     This condition, called hydatidiform mole, is usually associated with greatly increased production of the chorionic gonadotrophin. Many trophoblastic cells progress to invasion of the muscular part of the uterine wall and gain access to the blood stream. Their growth potential is retained, and they are seeded throughout the body, where are they present as one of the most florid and malignant cancers know. Oddly enough, this cancer is one of the most easily treated; the drug methoxrate is very often entirely curative.

     Many abnormalities in fetal development are of genetic origin. Some result from maternal disease, particularly German measles, in the period when the basic structure of the fetus is being laid down. Some result from the use of drugs by the mother in the same time period. The unfortunate thalidomide episode is an illustration of this, but it seems quite possible that almost any drug taken by the mother in the first three months of pregnancy may have serious consequences for the fetus.

     After three months, the fetus is relatively, but not quite, immune to diseases of the mother and to developmental abnormalities from the use of medicines or drugs. Diabetic mothers tend to have large babies, though the reason is unknown. Syphilis can cross the placenta, so babies of syphilitis mothers may be born with congenital syphilis. In certain circumstances (Chapter 30), maternal and fetal blood are in contact with each other. This, together with an immune discrepancy, may lead to erythroblastosis fetalis, more widely known as Rh incompatibility.

     The positive diagnosis of pregnancy can be missed if it is attempted too early. In one in a thousand women, most of the presumptive signs of pregnancy, but none of the probable or positive signs, may appear. The woman may be entirely convinced of the reality of her pregnancy, and so may her husband, if she has one, and her friends. Some cynics suspect that unmarried women with false pregnancy, pseudocyesis, may actually be using their condition as an argument in favor of marriage. In most cases however, the deception involved in pseudocyesis is self-deception, and the woman so afflicted may be considered to be severely neurotic, perhaps even psychotic.

     In the last three months of pregnancy, most women show a strong tendency to reabsorb salt from the tubules. This may be a result of the abundance of the steroid hormones, acting like aldosterone. It is usually compensated by an increase in the glomerular filtration rate. Though the mechanism of this increase is not known, in some women, the increase in glomerular filtration rate does not occur.

     In such women, there is a very strong tendency to retain salt and with it water. To a certain extent, this is normal (recall that blood volume and extracellular volume increase in pregnancy). When exaggerated, this tendency is very dangerous, and if unchecked, it results in eclampsia, in which convulsions and coma may lead to death. In its early stages, the condition, called pre-eclampsia, may lead to nothing more than a little weight gain in excess of normal, a little elevation of blood pressure, and some protein in the urine.

     The progression of the disease is usually halted by a low sodium diet, chlorothiazide, a sedative, and antihypertensive agents. The pregnant woman herself she notes a sudden weight gain. A woman who reports such a weight gain should be considered pre-eclamptic by her physician until proven otherwise.

     In the United States, 6-7% of a1l pregnant women develop toxemia of pregnancy, the term applied to both eclampsia and pre-eclampsia. One woman in three hundred pregnant women develops on to true eclampsia, and the death rate from eclampsia is one per two thousand pregnancies. Almost every one of these deaths is preventable if proper prenatal care is available.

     Low implantations of the ovum, as noted above, may result in early separation of the placenta during labor. Normally implanted placentas may also separate early, usually after the 28th week of pregnancy. Half such premature separations of the placenta occur before the first stage of labor. In some patients, vaginal bleeding indicates that placental separation has occurred, while in others, the bleeding is concealed (Figure 387).

     The infant, separated from the maternal blood supply, usually dies 50-60% of the time. The outlook for the mother is much better (0.5-5.0% mortality). Most maternal deaths are due to hemorrhagic shock, and most of them can be prevented by the immediate rupture of the fetal membranes. Following this, the bleeding uterine wall stanching to contract thus stanching blood losses, and normal labor may be induced, or a Caesarean section performed.

     The beginning of labor is marked by strong uterine contractions. Many women have irregular brief pains late in pregnancy, which are commonly mistaken for true labor pains and may be distinguished from them by the fact that the uterine contractions associated with them are weak. This is called "false labor."

     Strong, regular pains mark the beginning of true labor. These pains, which are much feared by many women, are not intolerable. Anesthesia during the first stage of labor is ill-advised, though sedative and analgesic drugs may relieve the pain to some extent without interfering with the normal processes of labor. Narcotics such as morphine should be used very cautiously, for they cross the placental barrier and depress the respiratory center of the infant.

     The type of anesthesia to be used in the second stage of labor, if any, varies with the physician, the mother, and current fashion. General anesthesia has its adherents, and some prefer local anesthetics, either pudendal block, caudal anesthesia, or spinal anesthesia. Hypnosis is favored by some; while some women, for obscure psychological reasons, prefer to experience the pain of natural childbirth.

     The head-down position, assumed in the description of normal labor, is the usual one, the back of the head (vertex) being the presenting part. The head-down position probably is due to the greater density of the head; the fact that the vertex is the usual presenting part may be due to the geometrical relationships between the lower part of the uterus and the head. Whatever the cause of normal presentation, it should be recognized that it is not so dominant as to prevent abnormal presentations. Thus, with the head down, the face or forehead may present first. The baby may lie horizontally, so that its shoulder rests on the cervix. The head may be up, the baby sitting, as it were on the cervix (breech presentation). Vertex and breech presentations present the fewest difficulties, but transverse presentations (the baby being horizontal) usually require rotation of the baby in the uterus or a Caesarean section.

     In any type of presentation, the uterine contractions of the first stage of labor may become weaker, rather than stronger, as time passes. Sometimes, the contractions originating at the top of the uterus are made useless by simultaneous contractions at the bottom. In either case, the first stage of labor is unduly prolonged and vigorous treatment is necessary. The administration of oxytocin may re-enforce uterine contractions when the first type of difficulty exists. Sedation sometimes helps in the second difficulty and Caesarean sections may be necessary.

     Prolongation of the first stage of labor is often a consequence of the mismatch between the fetal head (or other presenting part) and the pelvic outlet. A large head, a small outlet, or both may make normal delivery quite impossible. Proper prenatal care and pelvic measurements may prepare the physician and the mother for this type of difficulty, and it can sometimes be avoided. For example, a diabetic woman, whose baby tends to become oversized, should be considered as a candidate for early induction of labor or Caesarean section.

     Early induction of labor should only be carried out for specific medical reasons, not to suit the convenience of the pregnant woman, her husband, or the obstetrician. When it is indicated, two methods are available. Uterine contractions can be induced by oxytocin, or alternatively, the amniotic sac can be ruptured. The drainage of ammotic fluid shortens the muscular coat of the uterus and stimulates its further contractions.

     Caesarean section: In many cases (from 2-10% of all pregnancies), normal labor is impossible or dangerous. The delivery must then be made through an incision in the uterine wall. Caesarean section must be considered when there is placenta previa or premature separation of the placenta. In both cases, it may be life saving. Disproportion between the fetal head and the size of the pelvic opening is the most common indication, and uncontrollable pre-eclampsia and abnormal presentations such as shoulder or transverse which cannot be converted to normal presentations are also indications.

     The incision is usually made low in the abdomen, just above the bladder. The bladder is displaced downward and the uterine wall cut either transversely or longitudinally. The infant is delivered and the uterine wall closed in layers. This type of caesarean section, the low cervical, is the most favored today. The incision avoids contact with the peritoneal organs. The classical Caesarean section, higher in the abdomen, may result in adhesions of the bowel to the scar. Figure 388 shows the low cervical Caesarean section.

     After the second stage of labor, the uterus ordinarily contracts, the placenta is dislodged and expelled, and the open wound which was filled by the placenta is closed by contraction of the uterine muscle. Occasionally, this contraction is too weak to close the wound. This is especially likely after prolonged and difficult labor. Sometimes the placenta is only partially separated. In either case, serious hemorrhage may result. This is ordinarily avoided by giving pitocin until the placenta is expelled, and following it with ergot. This drug causes violent contraction of the uterus and usually controls bleeding quite well.

     The fetal circulation is adequate to sustain intra-uterine life. Blood leaving the left heart goes by way of the aortic branches to the usual organs. About 400 ml / min however, more than half the cardiac output, goes by way of the internal iliac arteries to the umbilical cord and passes to the placenta. This blood enters the chorionic villi, which are bathed in a flowing lake of maternal blood derived from the uterine artery. Diffusible materials exchange here.

     The blood returning from the placenta is not quite like arterial blood. The maternal blood lake does not flow fast enough to allow the fetal blood to come in equilibrium with maternal arterial blood. So far as carbon dioxide is concerned, the blood returning to the fetus is intermediate between maternal arterial and fetal arterial blood. Oxygen, however, may actually be higher in the blood returning from the fetus than the mother's arterial blood. This strange phenomenon depends on the fact that fetal hemoglobin differs from maternal hemoglobin, the affinity of fetal blood for oxygen being greater than that of adult hemoglobin.

     The umbilical vein joins the inferior vena cava at the level of the hepatic veins. The semi-arterial blood now enters the right atrium along with other blood from the great veins. Some of this blood passes into the left atrium at once through a hole in the interatrial septum, the foramen ovale. Its further course through the left ventricle and aorta is much the same as its course in the adult animal. On the other hand, the blood which goes to the right venticle has an unusual course.

     The lungs of the fetus are more or less collapsed: their vessels present a very high resistance to blood. Consequently, very little of the blood entering the right ventricle goes into the pulmonary circulation. Instead, as it leaves the pulmonary artery, it enters the aorta directly, by way of a large but short duct which connects these vessels, called the ductus arteriosus.

     Thus the fetus receives its nutrition from the mother by exposing its blood to maternal blood in the placenta, but the two bloods are not mixed. Instead of the usual circulatory arrangement in which the blood reaching the right heart goes to the lungs, and then to the left atrium, the fetus uses two by passes around the lungs: one direct communication from right to left atrium, and one direct communication from the pulmonary artery to the aorta. The fetal circulation is diagrammed in Figure 389.

     Separation of the placenta, tying the umbilical cord, or both suddenly deprives the newborn of a supply of oxygen, and more important, he has no way to get rid of carbon dioxide. The lack of oxygen, the rise in carbon dioxide, the passage through the vagina, and the sudden imposition of a variety of outside stimuli, including the famous slap, present an overpowering inspiratory stimulus.

     With the inspiratory gasp which follows, the lungs are stretched open. Their vascular resistance falls. Now, blood passing through the pulmonary artery returns to the left atrium by the pulmonary veins in the usual fashion. This raises the pressure in the left atrium and closes the foramen ovale, at first functionally, and later organically. At the same time, the arterial pressure rises, and the entire output of the right ventricle is now directed to the lungs. Within a few days the ductus arteriosus closes and the normal adult circulation is established. Figure 390 shows the vascular changes associated with the first breath.

     In some babies, the foramen ovale does not close. This produces an interatrial septal defect, which, if large, may require repair. Such a defect, which allows non-oxygenated blood to enter the systemic circulation, is a cause, though an uncommon one, of blue babies. This defect is illustrated in Figure 391.

     The ductus arteriosus sometimes remains open. When this occurs, some blood which leaves the left heart by way of the aorta may return to it by way of the pulmonary artery. The left heart must therefore continually handle an extra output. This is sometimes an indication for surgical correction (Figure 392).

Twins: One pregnancy in 80 is a twin pregnancy. Of these, three quarters are double ovum twins (fraternal). The rest are identical twins, derived from one fertilized ovum cleaving into two. Single ovum twins generally have a single placenta, but double placentas are not rare--about half of all double ovum twins have fused placentas.

     Twin pregnancies triple the incidence of toxemia of pregnancy, for unknown reasons. The growth of the double fetus is greater than that of a single fetus, so the uterus is distended faster and responds by contracting earlier. Three-quarters of all twins are born before term. The overstretched uterus contracts rather poorly during labor, which is often considerably prolonged. Premature separation of the placenta and placenta previa are much more common in twin pregnancies, and hemorrhage after delivery is also common.

     The twins, like the mother, have an unusual number of difficulties. In some cases, the umbilical cord of the second twin is compressed during the second stage of labor in the first with fatal consequences. Two twins may become "locked" if the first is a breech and the second a vertex presentation. Both may die, if this happens (See Figure 393). The risk of death for a twin is 4-5 times that of a single infant.

     Because the complications associated with twinning are numerous, the early diagnosis of twinning is of great importance. This can usually be made fairly easily in about 3 / 4 of the cases. Diagnosis by X-ray is too dangerous for routine use, and pelvic examination is usually helpful, but not infallible. Perhaps the most attractive way of making the diagnosis is by recording electrocardiograms from both twins, using electrodes on the maternal abdomen.

     Many women experience difficulty in nursing their children. Most of these are attributable to psychological problems, though in a few, the infant sucks weakly, so that there is not much stimulus for prolactin formation.

Continue to Chapter 29.