At 37 weeks, the cervix can remain dilated up to 3 centimeters for up to one month before active labor occurs, according to What to Expect. Cervical effacement and dilation can begin one month prior to delivery, or they can occur as quick as overnight.
Dilation of 2 centimeters does not indicate that labor is underway, but simply that the birth canal is readying itself for the passage of a baby, according to What to Expect. This initial stage of labor includes both effacement, or the thinning of the cervix, and dilation, the opening of the cervix. Around this time, a woman may notice her mucus plug, or the thick layer of mucus that protects the cervix during pregnancy, pass through her vagina. Once the mucus plug passes, which can occur any time between a couple of weeks to a couple of hours before labor begins, the cervix continues to thin and open further.
During prenatal checkups in the final month of pregnancy, a practitioner performs an exam of the cervix to determine the effacement and dilation of the cervix. Dilation is always measured in centimeters with 10 centimeters being completely open and ready for pushing, and 0 centimeters being completely closed.