Surrounded by membranes, the calf's forelegs and nose now protrude from the vulva. After the nose is exposed, the dam exerts maximum straining to push the shoulders and chest through the pelvic girdle. Once the shoulders have passed, the abdominal muscles of the calf relax and its hips and hind legs extend back to permit easier passage of the hip region. This ensures an oxygen supply for the calf during birth. Upon passage through the vulva, the umbilical cord generally breaks, respiration begins, filling the lungs with air and causing the lungs to become functional.
Delivery normally is completed in one hour or less in mature cows.
Special assistance is warranted if this stage goes beyond 2 to 3 hours. First-calf heifers can take 1 to 2 hours, or longer. Proper judgement should be used so that assistance is neither too hasty, nor too slow. The caruncleocotyledon, or button attachment between the uterus and placenta, relaxes and separates after parturition. The placenta is then expelled by continued uterine contractions.