During pregnancy, what causes uterine enlargement?

Uterine enlargement during pregnancy is primarily caused by hypertrophy, which refers to the increase in the size of the uterine muscle cells (myocytes). As pregnancy progresses, hormonal changes, particularly the increase in estrogen and progesterone, stimulate these muscle cells to grow and expand in response to the growing fetus. This adaptation not only accommodates the developing fetus but also prepares the uterus for labor and delivery.

In contrast, hypoplasia refers to underdevelopment or incomplete development of an organ or tissue, which does not occur in a normal pregnancy scenario. Atrophy involves the reduction in size or wasting away of an organ or tissue, which is not applicable during pregnancy since the uterus is actively growing. Neoplasia involves the abnormal proliferation of cells, often leading to tumors, and is unrelated to the natural physiological changes that occur during pregnancy. Therefore, hypertrophy is the correct mechanism explaining the uterine enlargement associated with a healthy pregnancy.