The reproductive organs of the cow also undergo discrete changes during gestation. The vulva enlarges, which becomes notable around the seventh month of gestation. The cervix remains tightly closed and is sealed by a mucus plug. Relaxation of the pelvic ligaments occurs gradually during gestation and becomes more noticeable with approaching parturition due to the hormone relaxin. Maintenance of pregnancy is largely dependent upon the proper balance of hormones maintained by interactions between the cow, the placenta and the fetus.
Progesterone has the dominant role for the maintenance of pregnancy.
Hence, the corpus luteum of the cow persists throughout gestation. The presence of the embryo in the uterus interrupts the estrous cycle. During this interruption the corpus luteum persists, releasing progesterone into both the blood and milk. Pregnancy also can be detected by rectal palpation. Most veterinarians prefer to palpate for pregnancy 45 to 60 days after breeding.