Targeted screening for short cervix in pregnancy will miss 40% of women

The cervix is the door to the uterus and, in normal circumstances, is long and closed until a woman goes into labor after 37 weeks. However, a short cervix is associated with preterm birth (or delivering before 37 weeks). A short cervix is generally defined as less than 2.5cm long. About 30% of women with a short cervix will give birth before 37 weeks. The American College of Obstetricians and Gynecologists recommends that OBs check the cervical length for every pregnant women who had a preterm birth in the past. For women with a history of preterm birth and a short cervix, progesterone (a hormone pill) placed in the vagina every day can reduce their risk of another preterm birth. Two recent randomized trials showed that vaginal progesterone treatment can reduce the risk of preterm birth in women with a short cervix, even if they don't have a history of preterm birth. It remains controversial if OBs should measure all pregnant women's cervical lengths (universal screening) or just women with a history of preterm birth.

A new study from Northwestern looked at the risk factors for short cervix, other than a history of spontaneous preterm birth, to see if there are other factors that can help OBs hone in on the women most at-risk of having a short cervix, thereby increasing the effectiveness of screening. They screening 18,250 women about 20 weeks into their pregnancies and 0.9% of them had a short cervix. The following factors were associated with a short cervix: Black race, Hispanic ethnicity, current tobacco use, prior preterm birth that was induced for medical reasons (not spontaneous), and a history of a cervical excisional procedure (like a LOOP or LEEP procedure for treatment of cervical pre-cancerous cells). Factors examined that were not associated with a short cervix included age, BMI, and conception by In Vitro Fertilization.

The authors then analyzed how effective cervical length screening would be if OBs only checked the cervical length of women with at least one risk factor. They found that specificity (true negative rate) increases from 62.8% for universal screening to 96.5%. However, the sensitivity (true positive rate) is 62.8%, which means that about 40% of women with a short cervix would not be screened and thus would be missed if OBs only screen women with a risk factor. 

The debate about whether to screen all pregnant women for short cervix continues. This study provides some risk factors that could increase the percentage of women screened who have a positive result, but will still miss a sizable proportion of women with a short cervix who have no risk factors.