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A: Dear Reader,

In first babies, effacement (the thinning of the cervix) comes first. The percentage is based on how thick the ring of cervical tissue is around the hole of exit it surrounds. One-hundred percent would be paper thin; 50 percent is not that impressive (sorry!). Usually when you get to 90 percent you can expect some dilation to begin.

In subsequent pregnancies, dilation will usually happen first. It is possible to be 3-4 cm dilated, but if there's little effacement, then this isn't very impressive either in terms of expecting imminent labor.

I have found that the most crucial measurement for expecting some action soon is effacement combined with the movement of the cervix to an anterior position (i.e., in line with the vagina). Before delivery becomes imminent, the cervix is posterior, or pointing toward your rear end on exam. The engagement of the fetal head will stretch the angle where the cervix joins the rest of the uterus, straightening it out so that the cervix pivots and aims more in line with the proverbial birth canal. When that happens, effacement can occur because the head is more directly pushing against the cervix, and effacement can progress.

Descent of the head into the pelvis—or engagement is also a reliable sign, when combined with an anterior cervix and effacement. Actual dilation is the least important when you have all of the other factors going for you.