As a mother approaches parturition, the fetus releases cortisol, a steroid hormone, into the bloodstream. This causes the placenta to produce estrogen, which triggers the uterine muscle wall to start contracting, according to the Purdue University website. The uterus also releases prostaglandins, a hormone-like substance that inhibits progesterone levels and induces contractions. Physical symptoms include the softening and swelling of the vulva, the dilation of the cervix, the expulsion of the mucus plug that has protected the fetus from bacterial infection, the rupture of the amniotic sac and a rise in temperature.
During the first stage of parturition, the fetus rotates from lying on its back with its feet pointing up inside the uterus to a right-side up position. The fetus' chin will recline against its forelegs, and its feet will point towards the cervix. If the fetus fails to rotate or moves into a position other than right-side up, dystocia or abnormal labor may result. The cervix relaxes and dilates and fetal tissue is exposed through the vulva. This stage should last for about six hours.
The next stage is the actual delivery of the newborn. Abdominal contractions increase and intensify, occurring every 3 to 5 minutes. The allantoic sac, or tissue that is part of the umbilical cord and placenta, breaks. The fetus and membranes then enter the birth canal. Depending on the animal, this stage can last from 30 minutes to 5 hours. For adult cows with previous deliveries, this stage passes rapidly unless some abnormal factor arises in the birthing process, such as a backwards calf, a large calf or twins. Cattle typically lie down during expulsion of the fetus.
During the final stage of parturition, the uterus expels the fetal membranes and begins to shrink. While contractions continue for about two days after birth to help the mother expel the placenta, the involution of the uterus can take as long as month. If placental membranes remain in the uterus, it can lead to infection and death. Although it was customary to manually remove any remaining membranes from the uterus, this method can damage the uterus and put future conception at risk. Veterinarians now use antibiotics to prevent infection and wait for placental membranes to fall out, which can take 4 to 7 days.