Question: how many centimeters before labor starts?
Answer: 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. Mild contractions begin at 15 to 20 minutes apart and last 60 to 90 seconds.