OBJECTIVE: To study the hypothesis that in women with threatened preterm labor, transvaginal measurement of cervical length helps distinguish between true and false labor. METHODS: We included in the study 112 women with singleton pregnancies presenting with regular and painful uterine contractions at 24-36 (mean, 32) weeks of gestation. Women in active labor, defined by the presence of cervical dilatation > or = 3 cm, and those with ruptured membranes were excluded. On admission to the hospital a transvaginal scan was performed to measure the cervical length. The primary outcome was delivery within 7 days of presentation. RESULTS: In 89 cases the cervical length was > or = 15 mm and only one of these women delivered within 7 days. In the 23 cases with cervical length < 15 mm delivery within 7 days of presentation occurred in 9 (39%) cases.Statistical analysis demonstrated that the only significant contributor in the prediction of delivery within 7 days was cervical length < 15 mm (PPV 95%,P < 0.0001).The variables like maternal age, gestational age, parity, previous history of preterm delivery, cigarette smoking, contraction frequency or use of tocolytics had poor significance. CONCLUSIONS: In women with threatened preterm labor, sonographic measurement of cervical length helps distinguish between true and false labor.