Final answer:
The cervix must dilate to about 10 centimeters in diameter before the baby can be delivered.
Step-by-step explanation:
For vaginal birth to occur, the cervix must dilate fully to 10 cm in diameter-wide enough to deliver the newborn's head. The dilation stage is the longest stage of labor and typically takes 6-12 hours. However, it varies widely and may take minutes, hours, or days, depending in part on whether the mother has given birth before; in each subsequent labor, this stage tends to be shorter.