The Physics of Labor

I had a sweet couple the other night whose labor reminded me of the physics involved in the work of labor.

Although she was only 2 centimeters dilated when she was admitted, her cervix was paper-thin and her contractions in a close, regular pattern.  I had a feeling she would go quickly and she did, mainly because of physics.

 The cervix, the opening to the uterus, begins thick and closed. The first action it takes, before ever opening up any appreciable amount, is to thin out, or efface. The muscles of the uterus pull upward toward the top and this pulls on and effaces the cervix.

A woman will usually be 90% or more effaced before she begins to dilate any significant amount. With a first baby the cervix will efface as thin as a piece of paper. Never again will the cervix be this thin.

                        Image via WikipediaThe action of the uterine muscles pulling upward on the cervix is similar to that of putting on a turtle-neck sweater. Most people don't begin by pulling open the turtle-neck as wide as they can and then pulling it over their head. No, they pull the sweater over their head and let the neck portion rest on top of their head, thick and closed.

As they pull the fabric of the sweater from the bottom, the neck portion gradually thins out. Once the neck has thinned out as much as it can, then the opening will begin to open up more and more and begin to slide over the person's head, until finally, their head slides through.

This is the same mechanism for labor. The uterine muscles pull on the cervix, as the baby's head, pressing against it, help it to thin out. Then when the cervix is as thin as it can get, it begins to dilate, or open up, more and more until the baby finally begins to descend through it.

While it is possible for woman who has had several babies to have a cervix that is still thick and dilated at the same time, that is not due to dilation as much to the elasticity of the cervix that multiple labors has caused, much like the stretchiness of a balloon which has been blown up over and over again.

When in labor, it is important to remember that progress in any number -- dilation, effacement or station, is still progress -- especially in early labor.   Don't be discouraged if those first early exams only show changes in effacement, those are necessary before the big numbers in dilation can be made possible!