The anatomical, physiological, and biochemical adaptations of pregnancy are profound. Many gestational changes begin soon after fertilization and continue throughout pregnancy. Equally astounding is the almost complete return to the prepregnancy state after delivery and lactation. Most pregnancy-related changes are prompted by stimuli provided by the fetus and placenta. Virtually every organ system undergoes alterations, and these can appreciably modify criteria for disease diagnosis and treatment. Thus, an understanding of pregnancy adaptations is essential to avoid misinterpretation. Moreover, some normal physiological changes can unmask or worsen preexisting disease.
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 thin-walled muscular organ of sufficient capacity to accommodate the fetus, placenta, and amnionic fluid. The total volume of the contents at term averages 5 L but may be 20 L or more! Thus, by the end of pregnancy, the uterus has achieved a capacity that is 500 to 1000 times greater than the nonpregnant state. The corresponding increase in uterine weight is such that, by term, the organ weighs nearly 1100 g.
Uterine hypertrophy early in pregnancy is probably stimulated by the action of estrogen and perhaps progesterone. Thus, similar uterine changes can be observed with ectopic pregnancy. But after approximately 12 weeks’ gestation, uterine growth is related predominantly to pressure exerted by the expanding products of conception.
Within the uterus, enlargement is most marked in the fundus. The extent of uterine hypertrophy is also influenced by the position of the placenta. Namely, the myometrium surrounding the placental site grows more rapidly than does the rest.
During pregnancy, uterine enlargement involves stretching and marked hypertrophy of muscle cells, whereas the production of new myocytes is limited. Fibrous tissue also accumulates, particularly in the external muscle layer, together with a considerable rise in elastic tissue content. The walls of the corpus considerably thicken and strengthen during the first few months of pregnancy but then gradually thin. By term, the myometrium is only 1 to 2 cm thick, and the fetus usually can be palpated through the soft, readily indentable uterine walls.
The uterine musculature during pregnancy is arranged in three strata. The first is an outer hoodlike layer, which arches over the fundus and extends into the various ligaments. The middle layer is a dense network of muscle fibers perforated in all directions by blood vessels. Last is an internal layer, with sphincter-like fibers around the fallopian tube orifices and internal cervical os. Most of the uterine wall is formed by the middle layer. Here, each myocyte has a double curve so that the interlacing of any two cells forms a figure eight. This crucial arrangement permits myocytes to contract after delivery and constrict penetrating blood vessels to halt bleeding.