Objective: The purpose of this study was to evaluate the effect of earliest previous spontaneous preterm birth (SPTB) gestational age on cervical length, pregnancy duration, and ultrasound-indicated cerclage efficacy in a subsequent gestation. Study Design: Planned secondary analysis of the National Institute of Child Health and Human Developmenttrial of cerclage for cervical length of <25 mm. Women with at least 1 previous SPTB between 17-33 weeks 6 days of gestation underwent serial vaginal ultrasound screening between 16 and 23 weeks 6 days of gestation; cervical length at qualifying randomization evaluation was used. Results: We observed a significant correlation (P = .0008) between previous SPTB gestational age and qualifying cervical length. In a linear regression model that was controlled for cervical length and cerclage, neither previous SPTB gestational age nor the interaction between cerclage and previous birth gestational age was significant predictor of subsequent birth gestational age. Conclusion: Although there is an association between previous SPTB gestational age and cervical length in women with a mid-trimester cervical length of <25 mm, there does not appear to be a disproportionate benefit of cerclage in women with earlier previous SPTB. © 2010 Mosby, Inc. All rights reserved.