Predictive value of cervical length for preterm birth in women with antepartum haemorrhage of unknown origin: a retrospective cohort study

This study aimed to determine whether transvaginal cervical length at presentation of antepartum haemorrhage of unknown origin (APHUO) can predict preterm birth (PTB), and to determine an optimal cut-off. This retrospective cohort included singleton pregnancies admitted with APHUO at 24–32 weeks of gestation to a Hong Kong tertiary hospital 2019–2025. Transvaginal cervical length was measured. The primary outcome was to evaluate the risk of PTB in women with or without short cervix (< 25 mm). Secondary outcomes included spontaneous PTB, delivery before 32 and 34 weeks, admission-to-delivery interval, and correlation with maternal and foetal outcome. Optimal cervical length cut-offs were determined according to gestational age and presence of other PTB risk factors. Among 321 women with APHUO, 62 (19.3%) had short cervix and 259 (80.7%) had normal cervical length. Short cervix was associated with earlier delivery (median 33 vs. 38 weeks), shorter admission-to-delivery interval (7 vs. 67 days), higher PTB rate ( p < 0.001). After adjustment for confounders, it remained strongly associated with PTB < 37 weeks [aOR 25.16 (95% CI 11.93–53.06)], < 34 weeks [41.67 (17.64–98.42)], < 32 weeks [60.39 (18.30-199.33)]. Cervical length and maternal history of PTB were associated with spontaneous PTB. Receiver operating characteristics analysis showed good predictive ability of cervical length on PTB, with optimal cut-off at 24 mm for 37 weeks with 95% specificity. Cervical length < 25 mm is a useful predictor for PTB in APHUO, and different cervical length thresholds should be applied according to the background PTB risk.