Final answer:
The second stage of labor begins when the cervix is fully dilated to about 10 centimeters and the baby is expelled from the uterus. The mother will experience strong contractions and actively push during this stage.
Step-by-step explanation:
The onset of the second stage of labor can be determined when the cervix is fully dilated to about 10 centimeters. This stage is characterized by the expulsion of the baby from the uterus. The mother may experience strong and frequent contractions, and she will actively push to aid in the delivery of the baby.