Final answer:
Heavy vaginal bleeding can occur due to a condition called placenta previa, where the placenta separates from the uterus and covers the cervix before the baby is delivered, causing antepartum hemorrhage.
Step-by-step explanation:
Heavy vaginal bleeding can occur if the placenta separates from the uterus before the baby is delivered, called placenta previa. Placenta previa is a condition in which the placenta partially or completely covers the cervix, which can lead to profuse vaginal bleeding after week 24 of pregnancy but prior to childbirth. It is considered a leading cause of antepartum hemorrhage. During the third stage of childbirth, the afterbirth, which includes the delivery of the placenta and associated membranes, occurs after the newborn has been delivered. If the placenta fails to be expelled naturally within a certain time frame, medical intervention may be required.