The greatest risk factor for preterm birth is a history of spontaneous preterm birth. As women in their first pregnancy have no birth history, preterm or otherwise, other methods are needed to determine if they’re at risk of delivering a preterm birth.
It’s very important that healthcare providers identify women with a prior cervical surgery so that they can monitor their cervical length by transvaginal ultrasound. And although it’s not as telling of a predictor of preterm birth as measurement by transvaginal ultrasound, checking a woman’s cervical length at the time of their anatomy ultrasound can be useful to identify those who have a cervix shorter than normal.
Cervical shortening and risk of preterm birth
At 32-34 weeks of pregnancy, nearly every pregnant woman’s cervix begins to soften and shorten, to prepare for birth. Women who aren’t at high risk of preterm birth don’t have much change in their cervical length until around this time.
The numbers
As with most measurements, what’s considered normal for cervical length isn’t one single measurement, but a series of measurements. The cervical length is a dynamic measurement that will vary slightly if performed throughout the course of pregnancy.
- Prior to pregnancy, most women have a cervix that is between 35and 55 mm in length.
- Between 16-24 weeks, the cervical length is usually between 35 and 48 mm. A short cervix is defined as being less than 25 mm in length before 24 weeks.
- By 32-34 weeks, the cervix normally starts to soften and shorten, so by the mid-to-late third trimester, it’s normal to have a cervical length of less than 30 mm.
Transvaginal ultrasounds can be helpful if performed between 16 to 24 weeks in predicting which women are at an increased risk for preterm birth. This is especially true for those with a history of a cervical surgery or a prior preterm birth. Transvaginal ultrasounds are not routinely performed as they are less predictive in women with no other risk factors. Measuring cervical lengths at various stages of pregnancy doesn’t guarantee protection from preterm labor, but it can help identify women that are at greater risk to be more closely monitored and those who are candidates for interventions allow for early and effective treatment, depending on what the provider determines is appropriate.
You should talk to your provider if you have any questions about cervical length throughout pregnancy.
Reviewed by the Ovia Health Clinical Team
Sources
- Shannon K Laughlin-Tommaso. “Cervical length during pregnancy: Why does it matter?” MayoClinic. Mayo Foundation for Medical Education and Research, May 2015. Web.
- “Cervical insufficiency and short cervix.” MarchofDimes. March of Dimes Foundation, Aug 15. Web.
- James P Nott, et al. “The structure and function of the cervix during pregnancy.” Translational Research in Anatomy. Vol 2:1-7. Web. Mar 2016.
