Which event is reliably expected during the second stage of labor?

During the second stage of labor, the primary event that is reliably expected is the delivery of the infant. This stage begins when the cervix is fully dilated to 10 centimeters and involves the mother actively pushing to help move the baby down the birth canal. This critical phase culminates with the birth of the baby as the contractions continue to assist in its descent and eventual delivery.

The other events mentioned do not occur during this stage. The breaking of the bag of waters, typically a sign that labor has begun, more commonly occurs in the first stage. Loss of the mucus plug tends to happen before labor begins, as the body prepares for delivery. The delivery of the placenta happens after the baby is born, indicating the end of the third stage of labor. Thus, the delivery of the infant is the defining outcome of the second stage, making it the correct answer.