The magic number is 10! Your cervix will be completely closed and it starts at 0 cm, however, during labor, it begins to dilate and it is only at 10 cm that you will be ready to give birth. Reaching from 0 to 10 might take a few hours for some mothers while some others might even take a few days to get to the 10 cm. For a vaginal birth, your cervix needs to be 10 centimeters dilated and 100% effaced.