The anatomical, physiological, and biochemical adaptations to pregnancy are profound. Many of these remarkable changes begin soon after fertilization and continue throughout gestation, and most occur in response to physiological stimuli provided by the fetus and placenta. Equally astounding is that the woman who was pregnant is returned almost completely to her prepregnancy state after delivery and lactation. Many of these physiological adaptations could be perceived as abnormal in the nonpregnant woman. For example, cardiovascular changes during pregnancy normally include substantive increases in blood volume and cardiac output, which may mimic thyrotoxicosis. On the other hand, these same adaptations may lead to ventricular failure during pregnancy if there is underlying heart disease. Thus, physiological adaptations of normal pregnancy can be misinterpreted as pathological but can also unmask or worsen preexisting disease.
During normal pregnancy, virtually every organ system undergoes anatomical and functional changes that can alter appreciably criteria for disease diagnosis and treatment. Thus, the understanding of these pregnancy adaptations remains a major goal of obstetrics, and without such knowledge, it is almost impossible to understand the disease processes that can threaten women during pregnancy.
In the nonpregnant woman, the uterus weighs approximately 70 g and is almost solid, except for a cavity of 10 mL or less. During pregnancy, the uterus is transformed into a relatively thin-walled muscular organ of sufficient capacity to accommodate the fetus, placenta, and amnionic fluid. The total volume of the contents at term averages approximately 5 L but may be 20 L or more. By the end of pregnancy, the uterus has achieved a capacity that is 500 to 1000 times greater than in the nonpregnant state. The corresponding increase in uterine weight is such that, by term, the organ weighs nearly 1100 g.
During pregnancy, uterine enlargement involves stretching and marked hypertrophy of muscle cells, whereas the production of new myocytes is limited. Accompanying the increase in myocyte size is an accumulation of fibrous tissue, particularly in the external muscle layer, together with a considerable increase in elastic tissue content. This network adds strength to the uterine wall.
Although the walls of the corpus become considerably thicker during the first few months of pregnancy, they then begin to thin gradually. By term, the myometrium is only 1 to 2 cm thick. In these later months, the uterus is changed into a muscular sac with thin, soft, readily indentable walls through which the fetus usually can be palpated.
Uterine hypertrophy early in pregnancy probably is stimulated by the action of estrogen and perhaps progesterone. The hypertrophy of early pregnancy does not occur entirely in response to mechanical distention by the products of conception, because similar uterine changes are observed with ectopic pregnancy (Chap. 19, Clinical Manifestations). But after approximately 12 weeks, the uterine size increase is related ...