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October 27, 2015

Due dates for pregnant women are often highly variable; only 5% of women deliver on their actual due date. Now a new study reports that measuring the length of a woman’s cervix with an ultrasound could help make that prediction more accurate (though not everyone agrees).

In the new study published in BJOG: An International Journal of Obstetrics & Gynaecology, researchers looked at five prospective studies that included, in total, 735 women with single-child pregnancies who had babies in the proper head-down position. In the studies, transvaginal ultrasound cervical length was used to predict the onset of labor.

The researchers reported that based on their analysis, when a woman’s cervix was more than 30 mm at her due date, she had less than a 50% chance of delivering within a week. But when the cervix was 10 mm or less, women had more than an 85% chance of delivering within a week.

This doesn’t necessarily add information for women who are not so far along, but the researchers say their study suggests that cervical length has some value in predicting when labor starts.

“It can enable better plans to be made regarding maternal transport,” the study authors write, suggesting some women who live far away may want to plan accordingly. “For pregnant women, this information may help them to arrange their social activities and deal with their anxiety.”

Physicians have been measuring women’s cervical length for a long time in order to assess if a woman is at risk for premature labor, the study authors note; a shorter cervix indicates labor may come sooner. The researchers say that since this method is used to predict early birth, some researchers have suggested that it could help predict on-schedule birth as well, and some have conducted research looking into this possibility.

However, not everyone agrees with its value. “I do not agree with the need, nor utility, nor benefit of preforming transvaginal ultrasounds at full-term to predict onset of labor,” says Fahimeh Sasan, an assistant professor of obstetrics, gynecology and reproductive science at the Icahn School of Medicine at Mount Sinai in an email to TIME. (Sasan was not involved in the study.) “Measuring a cervical length with transvaginal ultrasound between 37-39 weeks can be quite difficult because the cervix is not fully [or] optimally visible in all pregnancies via ultrasound at this gestation, hence making measurement not very reliable nor reproducible.” Sasan adds that 80% to 85% of healthy women with uncomplicated pregnancies deliver within seven days of their due date, “so the need for invasive transvaginal ultrasounds in my opinion is not clinically substantiated.”

The researchers acknowledge that not everyone agrees on the necessity, and other studies have differed in their findings. The authors say their own meta-analysis is limited due to the fact that sample sizes in the studies they used were small, and there are several other factors that play a role in when a woman gives birth.

Contact us at editors@time.com.

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