PREGNANCY

process and series of changes that take place in a woman's organs and tissues as a result of a developing fetus. The entire process from fertilization to birth takes an average of 266-70 days, or about nine months. (For pregnancies other than those in humans, see gestation.) the process and series of changes that take place in a woman's organs and tissues as a result of her having a developing fetus within her body. The entire process from fertilization to birth takes an average of 266 days, or about nine months. (For pregnancies other than human ones, see gestation.) The fertilization of an ovum (egg) by a spermatozoan usually occurs in one of the two fallopian tubes. As the fertilized egg, or zygote, moves down the tube toward the uterus, it undergoes repeated cell divisions. After several days, it consists of a ball of about 100 cells called a blastocyst, which differentiates into an inner group of cells that become the embryo and an outer ring of cells (the trophoblast) that becomes part of the placenta. About the seventh day after fertilization, the trophoblast implants in the uterine wall. The trophoblast invades the uterine lining, forming fingerlike projections that are surrounded by maternal blood vessels. This combination of trophoblastic and maternal tissue is the placenta. It brings the blood supply of the embryo into intimate-but not direct-contact with that of the mother, thereby enabling the exchange of nutrients and wastes between the embryonic and maternal circulations. The embryo itself is linked to the placenta by the umbilical cord, the vessels of which carry blood to and from the placenta. The embryo is surrounded and protectively cushioned by a fluid-filled amniotic sac. By the end of the first month, the embryo has reached a size of about 5 mm (0.2 inch) and most of its major organ systems have begun to form. These systems continue to develop during the second month, and it is during this period that sexual differentiation takes place. The limbs also appear during the second month. By the end of the second month, the major steps in organ development have taken place, and the embryo, though only about 3 cm (1.2 inches) long, is recognizably human in form. From this point forward, it is referred to as a fetus. During the remaining seven months of pregnancy, the fetus completes the maturation of its organ systems and grows dramatically in size. The most important symptom of pregnancy is the cessation of menstrual periods. Other indications are nausea, tender swollen breasts, and frequent urination. Tests for pregnancy are based on the detection of the hormone human chorionic gonadotrophin (hCG), which is produced by the developing placenta and prevents menstruation and the termination of the pregnancy. The body begins to produce hCG when the fertilized egg implants in the uterus; its concentration increases for the next 10 to 12 weeks, after which it decreases. The hormone is secreted into the pregnant woman's blood and is excreted in her urine; it can be detected in either of these fluids by immunoassay, which is a method of quantifying a chemical substance by means of an antigen-antibody reaction. This type of pregnancy test can be carried out as early as several days before the woman's first missed period. False results do occur, with false-negative results being more common than false positives, especially if the test is performed early in the pregnancy, before a sufficient amount of hCG is in the urine. Over-the-counter pregnancy tests are available for home use; however, they yield a higher incidence of false results than do laboratory tests. The most reliable method of detection is a radioimmunoassay of the woman's blood, which can establish pregnancy within a few days of conception. Pregnancy causes marked changes in a woman's body. The uterine wall thins out, and the uterus pushes upward into the abdomen until it presses on the diaphragm. (The uterus may lower slightly several weeks before delivery because the infant's head has dropped into the pelvis.) The breasts grow larger, and the pigmented area around the nipples darkens. The needs of the fetus put an extra burden on the mother's heart, and, by the 19th week, increasing amounts of blood make it necessary for the heart to do 30 to 40 percent more work than before pregnancy. The mother's blood pressure, however, should not rise. Uterine pressure on the bladder causes frequent urination, and the ureters and pelvis of the kidney enlarge and lose tonicity. During pregnancy the nutritional requirements of the mother increase. The need for protein almost doubles, and additional amounts of iron, calcium, and folic acid are required. A pregnant woman gains an average of 11 kg (24 pounds), of which only about 3.2 kg (7 pounds) is the fetus. Of the remaining weight, 1.8 kg (4 pounds) consists of amniotic fluid, placenta, and fetal membrane; 2.7 kg (6 pounds) is extra fluid throughout the body; 1.4 kg (3 pounds) is fat accumulation; and 1.8 kg is the increase in uterus and breast size. Various screening procedures are used during pregnancy to assess the health of the fetus. Ultrasound imaging is used routinely throughout pregnancy to monitor the structural and functional progress of the growing fetus. Other tests are used if the fetus is at risk for a particular health problem. For example, amniocentesis may be performed between the 15th and 17th weeks and involves the removal of amniotic fluid from the uterus. This fluid contains fetal cells that can be tested for certain genetic abnormalities and that can also be used to determine the sex of the fetus. Chorionic villi sampling is a procedure similar to amniocentesis that may be performed between the 8th and 12th weeks. Certain disorders may threaten a full-term pregnancy. For example, in abruptio placentae, which occurs in about one percent of all pregnancies, the placenta separates from the uterus before the birth of the fetus. This situation is very serious and requires the fetus to be delivered as soon as possible. Hypertensive disorders of pregnancy are a group of conditions distinguished by elevated blood pressure (hypertension) in the woman. The most common of these disorders is preeclampsia, which occurs in 5 to 10 percent of all pregnancies. The disorder is marked by a rise in blood pressure, protein in the urine, and edema; it does not develop until after the 20th week of pregnancy. Preeclampsia may progress rapidly into eclampsia, an extremely dangerous condition in which the woman experiences convulsions that can lead to coma and even death. Abortion is the termination of a pregnancy before the infant is able to survive outside the uterus and generally occurs before the 20th week. Abortion may be spontaneous (in which case it is called a miscarriage) or the result of medical intervention. Many conditions may cause miscarriages, but at least half of such spontaneous abortions are the result of a defect in the fetus. Abnormal changes in pregnancy Ectopic pregnancy An ectopic pregnancy is one in which the conceptus (the products of conception-i.e., the placenta, the membranes, and the embryo) implants or attaches itself in a place other than the normal location in the lining of the upper uterine cavity. The site of implantation may be either at an abnormal location within the uterus itself or in an area outside the uterus. Ectopic pregnancies outside the uterine cavity occur about once in every 300 pregnancies. They are one of the major causes of maternal deaths. Normally an ovum or egg passes from the ovary into the tube, is fertilized in the tube, and moves downward into the uterus. It buries itself in the lining of the upper part of the uterine cavity. It may pass farther down and attach itself to the lining of the mouth of the uterus (the cervix), creating a cervical pregnancy. These are rare and cause severe vaginal bleeding; the conceptus is expelled or discovered within a few months after implantation. If a conceptus attaches itself to the lower part of the uterine cavity, it is a low implantation. When a low implantation occurs, the placenta grows over the cervical opening, in a formation called a placenta praevia. This causes the woman to bleed, often profusely, through the vagina, because the placenta tears as the cervix begins to open during the latter part of pregnancy. When the fertilized egg implants in the narrow space or angle of the uterine cavity near the connection of the uterus with the fallopian tube, it is called an angular pregnancy; many angular pregnancies terminate in abortions; others go to term but are complicated because the placenta does not separate properly from the uterine wall after the birth of the baby. An angular pregnancy differs from a cornual pregnancy, which develops in the side of a bilobed or bicornate uterus. Implantation in the narrow part of the fallopian, or uterine, tube, which lies within the uterine wall, produces what is called an interstitial pregnancy. This occurs in approximately 4 percent of ectopic pregnancies. An interstitial pregnancy gradually stretches the wall of the uterus until-usually between the 8th and 16th week of gestation-the wall ruptures in an explosive manner and there is profuse bleeding into the abdomen. Most persons associate ectopic pregnancies with tubal pregnancies, because most ectopic pregnancies occur in the uterine tubes. The tube beyond the uterus has three parts: the isthmus, a narrow section near the uterus; the ampulla, which is wider and more dilatable; and the infundibulum, the flaring, trumpetlike portion of the tube nearest the ovary. A tubal ectopic pregnancy is designated by the area of the tube in which it is implanted. An isthmic pregnancy differs from one in the ampulla or infundibulum because the narrow tube cannot expand. Rupture of the affected tube with profuse intra-abdominal hemorrhage occurs early, usually within eight weeks after conception. Ampullar pregnancies, which are by far the most common, usually terminate either in a tubal abortion, in which the embryo and the developing afterbirth are expelled through the open end of the tube into the abdomen; by a tubal rupture; or, less commonly, by absorption of the conceptus. Sometimes the tube ruptures into the tissues attaching it to the wall of the pelvis, producing an intraligamentous pregnancy. Rarely, the embryo is expelled into the abdomen and the afterbirth remains attached to the tube; the embryo lives and grows. Such a condition is referred to as a secondary abdominal pregnancy. Primary abdominal pregnancies, in which the fertilized egg attaches to an abdominal organ, and ovarian pregnancies are rarer still. It is generally believed, but not proved, that most tubal pregnancies are caused by scars, pockets, kinks, or adhesions in the tubal lining resulting from tubal infections. The infection may have been gonorrhea; it may have occurred after an abortion, after the delivery of a baby, or after a pelvic surgical operation; or it may have been caused by appendicitis. Kinking, scarring, and partial adhesions of the outside of the tube may be the result of inflammation following a pelvic operation or of an abdominal inflammation. Tubes, defective from birth, may be too small for the passage of the conceptus or may be pocketed or doubled with one tubal half forming a blind pocket. There may be areas in the tubal lining that behave like the lining of the uterus (they show a decidual reaction that is conducive to implantation) so that they offer a favourable spot for the fertilized egg to implant. Pelvic tumours may distort the tube and obstruct it so that the conceptus cannot move downward. Theoretically, endocrine disturbances may delay tubal motility. Whatever the cause, when a tubal implantation occurs, it may be assumed that either migration of the fertilized egg within the tube was delayed by an extrinsic factor so that the egg grew to the point where it should implant or that the mechanism for implantation within the egg itself was prematurely activated in the tube. One or the other of these causative factors can sometimes be seen when a woman is operated upon for an ectopic pregnancy. In a great number of cases, however, no tube abnormality can be found. There is no satisfactory explanation for most abnormal implantations in the uterus, although defective uterine structure has been noted in some cases. Primary abdominal and ovarian pregnancies can best be explained by a mechanism in which the fertilized ovum is swept out of the tube by a reverse peristalsis of the tube, but it is quite possible that, in rare instances, the ovum and spermatozoa meet and fertilization and implantation take place within the abdomen. Ectopic pregnancy is frequently mistaken for other disorders. Typically, but not invariably, the woman who has an ectopic pregnancy in the ampullar part of the tube will have missed one or two menstrual periods. She need not have other symptoms of pregnancy. She has felt enough discomfort in the lower part of her abdomen to lead her to consult a physician. She has had recurrent episodes of rather light, irregular bleeding from the vagina. She has felt weak or faint at times. The signs of pregnancy are not likely to be present, and results of a pregnancy test are more often negative than positive. The physician, on pelvic examination, feels a tender, soft mass in one side of the pelvis. At this stage the differentiation must be made between an ectopic pregnancy and an intrauterine pregnancy with abortion, acute appendicitis, intestinal colic, inflammation of a fallopian tube, and a twisted ovarian tumour. Unless the diagnosis can be made, the patient continues to complain for several more days and then has a sudden severe pain and collapses from brisk bleeding within the abdomen. Sudden and acute abdominal pain and collapse due to severe hemorrhage are only rarely the first signs that something is amiss. If this does happen, it is usually because implantation has occurred in the isthmic portion of the tube and hemorrhage and tubal rupture occur simultaneously. More frequently, a woman has missed one menstrual period, has a sensation of pelvic pressure, feels that she must urinate, and collapses in the bathroom. She may be unconscious and pulseless from loss of blood when she arrives at the hospital. Interstitial pregnancies are often mistaken for intrauterine ones, but the patient has pain and may have intermittent vaginal bleeding. After several months she has sudden, severe pain, collapses from a massive intra-abdominal hemorrhage, and may die before surgical help can reach her. Most of the women who die from ectopic pregnancies do so from interstitial ones. Combined pregnancies, in which there is an ectopic pregnancy and a normal one in the uterus, or a fetus in each tube, have occurred and have compounded the difficulty in making a diagnosis. In a number of instances, the ectopic conceptus has been removed without complications, and the uterine fetus has progressed to term. Not all ectopic pregnancies end with a catastrophic hemorrhage and collapse. In a few instances tubal, abdominal, and broad ligament pregnancies have gone on until a living baby was obtained at the time of operation. In other cases the fetus died and, if very young, was resorbed; in others, when the fetus was larger, death was followed by absorption of the fluid in the sac, and the fetus was gradually converted into a more or less mummified mass. Some ectopic pregnancies of this type have caused no symptoms and have been carried by women for years. Undoubtedly many ectopic pregnancies that are in an early stage when they are expelled emerge through the open end of the uterine tube, are resorbed, and are never recognized. Once diagnosed, the treatment of ectopic pregnancies outside the uterine cavity is almost always a matter of prompt surgical intervention with proper attention to replacement of blood and fluid. Abortion Abortion is the termination of a pregnancy before the infant can survive outside the uterus. The age at which a fetus is considered viable has not been completely agreed upon. Many obstetricians use either 21 weeks or 400-500 grams (0.9-1.1 pounds) birth weight as the baseline between abortion and premature delivery, because few infants have survived when they weighed less than 500 grams at birth or when the pregnancy was of less than 21 weeks' duration. Generally speaking, the fetus has almost no chance of living if it weighs less than 1,000 grams (2.2 pounds) and if the pregnancy is of less than 24 weeks' duration. In one effort to resolve the matter, the American College of Obstetricians and Gynecologists has defined abortion as the expulsion or extraction of all (complete) or any part (incomplete) of the placenta or membranes, with or without an abortus, before the 20th week (before 134 days) of gestation. Early abortion is an abortion that occurs before the 12th completed week of gestation (84 days); late abortion is an abortion that occurs after the 12th completed week but before the beginning of the 20th week of gestation (85-134 days). In the past the word abortion usually meant to nonmedical persons the elective interruption of a pregnancy, whereas "miscarriage" indicated a spontaneous expulsion of the uterine contents. The term miscarriage is seldom used medically. Spontaneous abortion is the expulsion of the products of conception before the 20th week of gestation without deliberate interference. As a general rule, natural causes are responsible for loss of the pregnancy. An induced abortion is the deliberate interruption of a pregnancy by any means before the 20th week of gestation. In medical terminology an abortion may be therapeutic or elective (voluntary). A therapeutic abortion is the interruption of a pregnancy before the 20th week of gestation because it endangers the mother's life or health or because the baby presumably would not be normal. An elective abortion is the interruption of a pregnancy before the 20th week of gestation at the woman's request for reasons other than maternal health or fetal disease. Most abortions in the United States are performed for this reason. A spontaneous abortion usually passes through several progressive stages. The first stage is a threatened abortion in which a woman, known to be less than 20 weeks pregnant, notices a small amount of bloody discharge from her vagina and, perhaps, a few cramping pains in her uterus. By pelvic examination it is determined that her cervix has not started to open or dilate. Either the symptoms subside or the matter progresses to an inevitable abortion, in which there is increased bleeding, the uterine cramps become more severe, and the cervix, or mouth of the uterus, opens for the expulsion of the uterine contents. An inevitable abortion terminates either as a complete or an incomplete abortion, depending on whether or not all the products of gestation are expelled. The process may start abruptly with pain and profuse bleeding and be over in a few hours, or it may go on for days with only a modest loss of blood. Spontaneous abortions early in pregnancy tend to be complete. When the pregnancy is further advanced, it is more likely to be incomplete. Usually the physician removes the retained tissue in the uterus surgically when there is an incomplete abortion. If the fetus dies and is retained in the uterus for eight weeks or longer, the condition is referred to as a missed abortion. Women who lose three or more consecutive pregnancies of less than 20 weeks' duration are said to suffer from recurrent abortion. An infected abortion is an abortion associated with infection of the genital organs. Approximately 15 percent of all clinically evident pregnancies terminate in spontaneous abortion. A much higher rate of early pregnancy loss-more than 40 percent-is believed to occur. Some are lost so early that the woman and her physician are not sure whether she aborted or had a menstrual period that was slightly delayed, particularly heavy, and more painful than usual. The majority occur between the 6th and the 12th week after conception. Modifications in the abortion laws in several countries, including the United States, have greatly increased the number of requested abortions; it is believed that in some areas the number of abortions exceeds that of babies delivered alive. At least half of all spontaneous first-trimester abortions have been found by karyotyping (examination of chromosome characteristics) to have a chromosomal abnormality. Some of these genetic mistakes are caused by abnormal characteristics carried in the egg or sperm or by the failure of normal rearrangement of the chromosomes to occur after the egg and sperm unite. It has been shown in animals that disturbances in the transportation of the fertilized egg to the uterus may cause premature or delayed implantation of the conceptus; fertilized eggs that are too young or too old tend to abort. Inadequate secretion of the ovarian hormones estrogen and progesterone, needed for the development of the newly fertilized egg, may cause failure of the lining of the uterus and its secretions to sustain the young embryo. Later, failure of the placenta to take over the hormone-producing function of the ovary may adversely affect the growth of the uterus and its contractility. X rays in large doses, radium, and certain drugs may cause abortion because they damage embryonic tissues. Abnormal development of the mother's uterus may make it impossible for it to retain the pregnancy. Late abortion is sometimes caused by the weakness of the cervix or by fetal death following knotting of the umbilical cord. Uterine tumours may cause abortion because they increase uterine irritability or create an unfavourable environment for embryonic growth. In most instances in which psychological factors allegedly caused an abortion, examination of the baby and of the afterbirth have shown defects in one or both that had occurred before the mother had suffered her emotional disturbance. Physical injury to the mother is a causative factor in only one in a thousand abortions. Abortions thought to be caused by automobile accidents, falls, kicks, and so forth are often the result of deleterious changes in the fetus and sac that occurred before the injury. Systemic diseases may play a role in causing an abortion. This is particularly true of acute infectious diseases with high fever and bacteria in the bloodstream, or of diseases such as pneumonia, in which there is a marked reduction in the supply of oxygen to the fetus. Heart disease, kidney disease, diabetes, high blood pressure, and other chronic diseases may be associated with premature birth and fetal death after the 21st week but do not ordinarily cause abortions. Perhaps 3 percent of threatened abortions are prevented by rest and hormonal therapy. Most abortions are inevitable because the fertilized egg is abnormal; these cannot be controlled medically. Many women who suffer from recurrent abortion respond well to treatment; in some of these cases corrective surgery is necessary. An early spontaneous abortion without infection is rarely followed by ill health when the affected person receives proper medical treatment. Infected abortions, many the result of elective interruptions of pregnancy, have caused chronic pelvic distress and, in some cases, sterility. Additional reading General texts include Human Embryology, 4th ed. by W.J. Hamilton and H.W. Mossman (1972); and Williams Obstetrics, 19th ed. by F. Gary Cunningham et al. (1993), a standard textbook in the field. Additional information may be found in the following specialized texts: Irwin R. Merkatz and Joyce E. Thompson (eds.), New Perspectives on Prenatal Care (1990); Robert K. Creasy and Robert Resnik (eds.), Maternal-Fetal Medicine: Principles and Practice, 3rd ed. (1994); Burwell and Metcalfe's Heart Disease and Pregnancy, 2nd ed. by James Metcalfe, John H. McAnulty, and Kent Ueland (1986); and Richard S. Abrams, Handbook of Medical Problems During Pregnancy (1989). John W. Huffman The Editors of the Encyclopdia Britannica