- Objective The aim of this study was to determine whether the risk of early spontaneous preterm delivery (PTD) in asymptomatic women with a sonographic cervical length of ≤ 15 mm in the mid-trimester changes as a function of gestational age at diagnosis. Methods This cohort study included 109 asymptomatic patients with a sonographic cervical length of ≤ 15 mm diagnosed at 14–24 weeks of gestation. Women with a multifetal gestation, cerclage and a cervical dilatation of > 2 cm were excluded. The study population was stratified by gestational age at diagnosis (< 20 weeks vs. 20–24 weeks) and by cervical length (≤ 10 mm vs. 11–15 mm). The primary outcome variables were PTD at < 28 and < 32 weeks of gestation and the diagnosis-to-delivery interval. Results The median gestational age at diagnosis of a short cervix before 20 weeks and at 20–24 weeks was 18.9 and 22.7 weeks, respectively. Women diagnosed before 20 weeks had a higher rate of PTD at < 28 weeks (76.9% vs. 30.9%; P < 0.001) and at < 32 weeks (80.8% vs. 48.1%; P = 0.004), and a shorter median diagnosis-to-delivery interval (21 vs. 61.5 days, P = 0.003) than those diagnosed at 20–24 weeks. The rate of amniotic fluid sludge was higher among patients diagnosed with a short cervix at < 20 weeks of gestation than in those in whom it was diagnosed between 20 and 24 weeks (92.3% vs. 48.2%; P < 0.001). Conclusions Asymptomatic women with a sonographic cervical length of ≤ 15 mm diagnosed before 20 weeks of gestation have a dramatic and significantly higher risk of early preterm delivery than women diagnosed at 20–24 weeks. These findings can be helpful to physicians in counseling these patients, and may suggest different mechanisms of disease leading to a sonographic short cervix before or after 20 weeks of gestation. Copyright © 2010 ISUOG. Published by John Wiley & Sons, Ltd.