During childbirth, what signifies that the second stage of labor has begun?

The second stage of labor begins when the cervix is fully dilated to 10 centimeters. This stage is characterized by the mother's ability to push, as the baby's head descends into the birth canal. The transition from the first stage of labor, during which the cervix progressively dilates, to the second stage is marked specifically by this full dilation.

While strong contractions and the rupture of membranes may occur prior to the start of this second stage, they do not define it. The appearance of the baby's head can also be a visual cue indicating that the delivery is imminent, but the actual beginning of the second stage is officially recognized at the point of complete cervical dilation.