Cervical Length & Preterm Birth Risk (SMFM/ACOG Thresholds)

Transvaginal cervical length (CL) is the strongest single ultrasound predictor of spontaneous preterm birth. This tool applies published absolute-length management thresholds rather than a proprietary numeric percentage risk. In the first trimester (11-13 weeks), Greco et al. (Prenat Diagn 2011; Fetal Diagn Ther 2012) showed that a shorter endocervical length combined with maternal characteristics detects roughly 55% of spontaneous deliveries before 34 weeks at a 10% false-positive rate, but there is no widely standardized absolute-length treatment cutoff at this gestation — a short first-trimester measurement mainly signals the need for dedicated mid-trimester surveillance. In the mid-trimester (16-24 weeks), the Fetal Medicine Foundation cervical-length screening programme (To et al.) established that CL distribution predicts spontaneous preterm birth, with roughly 25 mm approximating the 10th centile at 18-24 weeks. SMFM Consult Series #70 (2024) recommends vaginal progesterone for a singleton pregnancy without prior spontaneous preterm birth when CL is 20 mm or less before 24 weeks (strong recommendation), with progesterone reasonable to consider by shared decision-making at 21-25 mm; routine cerclage is not recommended for painless shortening of 10-25 mm without a prior spontaneous preterm birth. In women with a prior spontaneous preterm birth, serial CL surveillance from 16-24 weeks with a threshold of 25 mm or less prompts a discussion of ultrasound-indicated cerclage in addition to progesterone, per ACOG/SMFM pathways. The Fetal Medicine Foundation also offers a certified mid-trimester history-plus-cervical-length percentage risk calculator; its full coefficient set is not publicly available and is not reproduced here — this tool applies the openly published absolute thresholds and management pathways only.