Ultimately, cervix dilation is necessary for labor to progress. If the cervix doesn't open, then the baby can't be born vaginally. But is cervical dilation necessarily an early sign of labor?
Not necessarily. A woman can remain 4cm dilated for weeks before true labor begins or can go from no dilation to natural childbirth within a few short hours. The degree of cervical dilation prior to the onset of labor is not a reliable indicator of when true labor will begin. Women can dilate several centimeters weeks early, contemplating the arrival of a preterm baby, and then remain at that same degree of dilation for weeks or go from zero dilation to having a baby in mere minutes.
The following cervical dilation chart illustrates the diameter of the cervix as it opens from 1 to 10cms. 1cm roughly equates with a Cheerio, 3 cm with a slice of banana, 4cm with a Ritz Cracker, 7cm with the top of a soda can, and 10cm with a bagel.
As cervix dilation itself is not a reliable predictor of when labor will begin, knowing the degree of cervical dilation in advance of active labor is of little benefit. It also carries unnecessary risks, the first of which is introducing infection. It also increases the risk of starting preterm labor.
Internal exams are not recommended for the above reasons until after 41 weeks when potentially facing a postdate induction. Then, cervical dilation can provide valuable information on whether induction is likely to be successful at that point.
Page Last Modified by Catherine Beier, MS, CBE
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