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The following is a summary of “Impact of cut-off values on the prevalence of short cervical length in pregnancy,” published in the September 2024 issue of Obstetrics and Gynecology by Breuking et al.

Previous research has suggested that employing a cut-off value of ≤30 mm for short cervical length (CL) might introduce selection bias and alter the distribution of CL measurements. This study aims to investigate how varying cut-off values affect the distribution of CL measurements and the incidence of short CL.

This secondary analysis utilizes data from the Quadruple P (QP) Screening study, a prospective cohort study involving low-risk women with singleton pregnancies who underwent a fetal anomaly scan at 18–22 weeks of gestation, including CL measurement. Participants with a short cervix, defined as ≤35 mm, were invited to the QP trial—a randomized controlled trial (RCT) comparing progesterone and cervical pessary for preterm birth prevention. If they declined the RCT, those with a CL ≤25 mm were advised to use progestogen. The primary objective was to evaluate the normal distribution of CL across the cohort and assess the incidence of short CL using cut-off values of ≤35 mm and ≤25 mm. Simulated normal distributions of CL were based on the mean and standard deviation of the original data, and the Kolmogorov-Smirnov test was employed to assess these distributions. Additionally, sonographers provided qualitative feedback via a questionnaire regarding their measurement practices around the cut-off values.

The cohort comprised 19,171 participants with a mean CL of 43.9 mm (±8.1 SD). The CL measurements deviated significantly from a normal distribution (P < 0.001). A total of 1,852 (9.7%) patients had a short CL ≤35 mm, considerably fewer than the 2,661 (13.9%) expected based on the simulated normal distribution (p < 0.001). The incidence of short CL ≤25 mm was 238 (1.2%), higher than the 177 (0.9%) anticipated from the normal distribution (p = 0.003). The most significant distribution discrepancy occurred around the 35–36 mm threshold. Sonographer feedback indicated that cut-off values did not influence their decision-making regarding study participation or treatment with progesterone.

This study underscores that the choice of cut-off value for short CL significantly impacts the incidence and distribution of CL measurements. Using a cut-off of ≤35 mm for study inclusion results in a lower-than-expected incidence of short CL, while a ≤25 mm cut-off for progestogen treatment leads to a higher frequency of measurements below this threshold. These findings highlight the potential for selection bias when applying specific cut-off values and suggest that careful consideration is needed when such values influence clinical decision-making.

Source: sciencedirect.com/science/article/abs/pii/S0301211524004858