As pregnancy progresses, the uterus undergoes gradual enlargement to permit expansion of the fetus, but its muscular walls remain quiet to prevent premature expulsion. Three phases can be identified in the adaptation of the uterus to accommodate pregnancy proliferation, growth and stretching. Uterine proliferation occurs before blastocyst attachment and is promoted by high progesterone levels. Characteristic changes of the inside lining of the uterus initiated by progesterone are increased blood supply, growth and coiling of the uterine glands, and leucocyte infiltration. Uterine growth starts after implantation.
Uterine growth includes muscular hypertrophy and an extensive increase in connective tissue.
Modification of the connective tissue is important both during uterine adaptation to the growing fetus and during involution after calving. The structural changes which take place in the pregnant uterus are reversible but are restored at different rates after parturition. When the mother recognizes the presence of a viable fetus in the uterus, the corpus luteum persists as the corpus luteum of pregnancy and subsequent estrous cycles are suspended. The corpus luteum of pregnancy in the cow persists at a maximal size and continues to produce progesterone throughout pregnancy. However, from about day 150 to day 250 of gestation, the fetal-placental unit is usually capable of sustaining progesterone production and pregnancy should the corpus luteum regress.