PURPOSE: To assess the implications of spontaneous changes in the length and configuration of the gravid cervix during a single ultrasound (US) examination and to correlate specific cervical dimensions with pregnancy outcome. MATERIALS AND METHODS: Sonograms in 27 pregnant patients with a spontaneously changing cervix were studied prospectively. The length and width of cervical funneling and the length of intact cervix caudal to the funneling were measured when the cervical dimensions were most normal and most abnormal. Sonographic measurements were correlated with clinical and delivery data. RESULTS: Twenty patients delivered preterm, although only six delivered within a week of the US examination. Wider funneling of the internal os and a shorter segment of intact cervix caudal to the funneling both correlated with an increased likelihood of preterm delivery. CONCLUSION: Most patients with a spontaneously changing cervix deliver preterm. Measurements obtained when the cervix appears most abnormal are most predictive of early delivery.