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Re: Short cervix: is a follow‐up measurement useful?

We commend the efforts of Fox et al.1 and agree with them that, in patients diagnosed with a short cervix, followup cervical length measurement is a strong predictor of preterm delivery. However the authors make no mention of the role of bacterial vaginosis, internal os dilatation and influence of the degree of cervical curvature on cervical length. Bacterial vaginosis is an independent risk factor for preterm delivery, affecting 6–32% of pregnant women2. So screening for bacterial vaginosis in pregnant women and subsequent antibiotic treatment can effectively prevent preterm labor3. Would it be possible for Fox et al. to comment on the incidence of bacterial vaginosis in their population? Fox et al.1 measured cervical length from the internal os to the external os in a straight line. However cervical length is influenced by the degree of cervical curvature. The disparity in measurements between the internal and external os taken as a straight line or along the cervical canal increases with cervical length. In one study, curvature of the cervix was observed in 48% of 301 women, and increased with cervical length from 0% at less than 16 mm to 25% at 16–25 mm and 51% at 26–55 mm4. Gramellini et al.5 measured cervical length in a straight line, if the cervix did not show any curvature. In the presence of cervical curvature, the measurement was broken down into two or more segments. By breaking down the measurement like this and then combining these segments together, they obtained biometric data that were certainly higher than those provided by a single straightline measurement, indicating that the latter approach underestimates the actual length of the cervical canal in the event of curvature. Dilated internal os and funneling of the internal os have been recognized as important contributors to preterm delivery, particularly in the late second and very early third trimester. Several investigators have shown an increased risk of preterm delivery among women with dilated internal os detected on transvaginal sonography6,7. The internal os is open in about 25–33% of patients at high risk for preterm delivery6,8. The sonographic finding of premature dilatation of the internal os > 1.3 cm in the second trimester is associated with a high rate of delivery at < 34 weeks6. We therefore seek clarification from Fox et al. and hope that further discussion will help in facilitating the provision of obstetric care.