Understanding Cervical Dilation in Labor: What to Expect

Question
Hello Dr.,

I am about 3 months pregnant with my first baby and have been reading widely on pregnancy, labor and delivery.  I keep reading that every woman is different and will have a different length of labor, and different amounts of pain, etc.  

One thing that confuses me, though, is that there seems to be an unwritten law that says EVERY woman MUST be dilated to 10cm. before she can start to push.  This doesn't make logical sense to me.  I am tall (5'9.5"), and slender (125 lbs before pregnancy) and I can't believe that my cervical dilation is going to behave exactly the same as an average or petite woman's or someone who is heavier or larger framed.

Right now I am planning on just pushing when my body starts pushing on its own whether I want it to or not. I've read many women's description of this experience, and it makes sense to me.

What does the medical literature say about variations in maximal cervical dilation? And when is the optimal time to start pushing?

Thank you,
Lori Schaffer

Answer
The cervix of EVERY woman is fully dilated when it reaches 10cm in diameter. We do not encourage women to start pushing until the cervix is fully dilated because if there is any cervix left and she starts pushing, the cervix can tear, leading to intense hemorrhage and a difficult repair. Do not start pushing until your care provider tells you that you are fully dilated. If this was your third or fourth pregnancy (or more), your cervix can become very friable and soft and these women can sometimes push the baby out without tearing the cervix if there still remains a tiny "lip" of cervix. However, in the first delivery, it would be disastrous to start pushing too early. Full dilatation is 10cm in tall women, short women, obese women, or thin women. Good luck.