Gestational age at cervical length measurement and incidence of preterm birth

Objective: To estimate the risk of spontaneous preterm birth based on transvaginal ultrasound cervical length and gestational age at which cervical length was measured. Methods: Women at high risk for spontaneous preterm birth and with transvaginal ultrasound cervical length measurements between weeks 12 and 32 were identified at one institution between July 1995 and June 2005. Inclusion criteria for women at high risk for spontaneous preterm birth were prior spontaneous preterm birth at 14 to 35 weeks, cone biopsy, mullerian anomaly, or two or more dilation and evacuations. Women with multiple gestations, cerclage, indicated preterm birth, or fetal anomalies were excluded. Logistic regression was used to estimate the spontaneous preterm birth risk before 35, 32, and 28 weeks. Results: Seven hundred five women received 2,601 transvaginal ultrasound measurements for cervical length. The incidences of spontaneous preterm birth before 35, 32, and 28 weeks were 17.7, 10.6, and 6.7%, respectively. The risk of spontaneous preterm birth before 35 weeks decreased by approximately 6% for each additional millimeter of cervical length (odds ratio 0.94, 95% confidence interval, 0.92-0.95, P=.001) and by approximately 5% for each additional week of pregnancy at which the cervical length was measured (odds ratio 0.95, 95% confidence interval 0.92-0.98, P=.004). Similar results were obtained for spontaneous preterm birth before 32 and 28 weeks. Conclusion: Gestational age at which transvaginal ultrasound cervical length is measured significantly affects the calculation of risk of spontaneous preterm birth. The spontaneous preterm birth risk increases as the length of the cervix declines and as the gestational age decreases. These spontaneous preterm birth risks are important for counseling and management for women with various degrees of short cervical length at different gestational ages.