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Question #41412 posted on 12/17/2007 3:01 a.m.

Dear 100 Hour Board,

I'm running into a research block, so I thought I'd try here. I'm pregnant and I've been doing lots of research on pregnancy and labor and such. There is one thing I can't find: when did 10cm become "the number" for cervical dilation? I'm assuming some study was done and 10cm became the "average" at some point, but I can't find the source of this mysterious study.

I ask because I'm sure some women's bodies would be fully dilated at 9cm and some wouldn't be ready until 11 or even 12. I'm just looking for a source on the matter. Maybe it's just one of those medical myths that became "fact" and no one questions it, because no one seems to cite it.

- From the Source

A: Dear From the Source,

From what I read, it sounds as though 10 centimeters is considered to be full dilation for all women regardless of their size. In one online article that I read, the authors (an MD and an RN) claim that 10 centimeters is considered to be "full dilation" because that measurement is the average diameter of a full-term fetus's head. In all of the many articles I read, though, no one seemed to say what happens if your child has an abnormally large head. If you're concerned about this, I'd ask your doctor.

As for your question about who first came up with 10 centimeters as the standard measurement, I've looked and can't find a name or a date to put with it. This makes sense, though, if you consider that the standard for full dilation is based off of the diameter of a standard baby's head. This idea seems to be based too much on common sense to be considered particularly noteworthy.