Final answer:
The third stage of labor starts after the baby is delivered and involves the passage of the placenta.
Step-by-step explanation:
The third stage of labor begins after the baby delivers. There are three stages to labor. During stage one, the cervix thins and dilates, which is necessary for the baby and placenta to be expelled during birth, and the cervix will eventually dilate to about 10 cm. Stage two involves the baby being expelled from the uterus, where the uterus contracts and the mother pushes, compressing her abdominal muscles to aid the delivery. The final stage three is characterized by the passage of the placenta after the baby has been born and the organ has completely disengaged from the uterine wall. This last stage occurs after the birth of the baby, and if labor should stop before reaching stage two, synthetic oxytocin, known as Pitocin, can be administered to restart and maintain labor.