Final answer:
The first part of the baby's head distending the vagina is referred to as the expulsion stage or the second stage of labor.
Step-by-step explanation:
When the first part of the baby's head distends the vagina, this is referred to as expulsion stage or the second stage of labor. During this stage, the cervix is fully dilated and the baby is pushed out of the uterus and through the birth canal. The baby's head, in a vertex presentation, exits the birth canal first, followed by the rest of the body.