Final answer:
Crowning refers to the moment in the expulsion stage of childbirth when the baby's head becomes visible at the vaginal introitus, typically in an occiput anterior vertex presentation.
Step-by-step explanation:
During a delivery, when the fetal scalp is visible at the vaginal introitus, it is called 'crowning.' This term refers to the moment in the expulsion stage of childbirth when the baby's head first appears at the vaginal opening. The expulsion stage, which is the second stage of labor, begins when the fetal head enters the birth canal and ends with the birth of the newborn. This stage can last from a few minutes to up to 2 hours and is dependent on various factors including the position and orientation of the fetus. Particularly, the occiput anterior vertex presentation is the most efficient for vaginal birth, where the fetus faces the maternal spinal cord and the occiput, being the smallest part of the head, exits the birth canal first.