Doula

Answer: D - The uterus is an involuntary muscle that contracts under the influence of a certain level of the hormone oxytocin. Oxytocin causes smooth muscle fibers to contract. The top portion of the uterus is the active component - it tightens and pushes down on the baby. The bottom part of the uterus passively stretches with each contraction, thinning the cervix as it is pulled open. The baby is not so much pushed out of the uterus as the uterus is pulled up over the baby's head. Once the baby's head passes the cervix, the uterine contractions, along with the abdominal muscles and active bearing down on the part of the mother, work to push the baby out of the birth canal.