Question: how many centimeters are you when labor starts?
Answer: The First Stage of Labor. The first stage is the longest part of labor and can last up to 20 hours. It begins when your cervix starts to open (dilate) and ends when it is completely open (fully dilated) at 10 centimeters. When the cervix dilates from 0 to 3 or 4 centimeters, contractions get stronger as time progresses ...