Final answer:
The fourth stage of labor is the delivery of the placenta and associated membranes, also known as the afterbirth.
Step-by-step explanation:
The fourth stage of labor following the vaginal delivery of a 6-lb baby girl and the placenta is the delivery of the placenta and associated membranes, also known as the afterbirth.
After the baby is born, the myometrium continues to contract, which causes the placenta to detach from the uterine wall and be expelled through the vagina.
During this stage, the uterus undergoes further contractions to reduce blood loss from the site of the placenta.