(889) Optimal Timing of Cervical Length Screening Before 24 Weeks in Twin Pregnancies

Sonographic cervical length (CL) is a valuable screening test for preterm birth (PTB). In twin pregnancies, vaginal progesterone and cervical cerclage may reduce PTB risk in patients with CL≤25mm and ≤15mm, respectively, if identified before 24 weeks. However, the optimal timing for CL measurement remains uncertain. While later measurement increases the overall detection of short cervix, it may delay the diagnosis and timely intervention for those whose cervix shortened earlier. This study aimed to determine the gestational week at which CL measurement provides the optimal balance between overall detection and timely diagnosis of a short cervix.

Study Design:

A retrospective study of patients with twin pregnancies followed at a single center (2012-2024). Patients underwent routine serial CL measurements every 2-3 weeks from 16-18 weeks until 28-30 weeks. For each week between 16 and 23 weeks, we calculated: (1) the cumulative proportion of patients with CL≤25mm and ≤15mm detected at that week; and (2) the proportion whose diagnosis of a short cervix would have been delayed by ≥2 weeks (who might have missed the opportunity for a timtely intervention). The optimal week was defined as the one achieving the best trade-off between overall detection and delayed diagnosis.

Results:

A total of 1,571 patients underwent 4,828 CL measurements between 16-24 weeks. The cumulative proportion with CL≤25mm increased from 1% at 16 weeks to 11% at 24 weeks, while the corresponding increase for CL≤15mm was from 0% to 6% (Figure 1). CL measurement at 23 weeks offered the optimal balance between overall detection and delayed diagnosis (Figure 2). When considering a strategy of two CL measurements, the best combinations were 19 and 23 weeks for CL≤25mm, and 21 and 23 weeks for CL≤15mm.

Conclusion:
In twin pregnancies, a single CL measurement at 23 weeks optimizes the detection of a short cervix while minimizing delayed diagnosis. A two-step approach with an earlier scan at 19 or 21 weeks may further improve the timely diagnosis of a short cervix and the opportunity for intervention.