Labor is a series of continuous, progressive contractions of the uterus that help the cervix dilate and efface (thin out). This lets the fetus move through the birth canal. Labor usually starts two weeks before or after the estimated date of delivery. However, the exact trigger for the onset of labor is unknown. During active labor, your cervix will dilate from 6 centimeters (cm) to 10 cm. Your contractions will become stronger, closer together and regular. Your legs might cramp, and you might feel nauseated. You might feel your water break — if it hasn't already — and experience increasing pressure in your back. There are several signs that labour might be starting, including contractions or tightenings and a "show" when the plug of mucus falls out.