Myocyte hypertrophy and proliferation in the myometrium were observed in mice during the first pregnancy and involution of the myometrium was completed by day 10 postpartum. Parameters of all structures returned to values observed in intact uterus. During the third pregnancy, the processes of myometrium hypertrophy were similar to those during the first pregnancy, but the intensity of proliferation was higher against the background of poorer vascularization, which led to destruction of some myocytes. Postpartum involution of the myometrium in these mice was not completed by day 10 after delivery. We observed hypertrophy and proliferation of some myocytes followed by their destruction, decrease in vessel number, and many-fold decrease in collagen content in the myometrial interstitium. The number of fetuses in the litter decreased after multiple pregnancies.