We evaluated whether the performance of an amniocentesis in women with a dilated cervix presenting at less than 26 weeks and subsequently managed by a physical exam-indicated cerclage increases the risk of spontaneous preterm birth (PTB) less than 28 weeks. Women between 15 (0)/ (7) to 25 (6)/ (7) weeks' gestation with a dilated cervix (1 to 4 cm) were identified. Multiple exclusion criteria were designated. The primary outcome was PTB less than 28 weeks. One hundred twenty-two women with a dilated cervix between 15 and 25 (6)/ (7) weeks gestational age were identified. Twenty-four (20%) of these had an amniocentesis performed. The unadjusted rate of PTB < 28 weeks differed between women who underwent amniocentesis compared with those who did not (58% versus 34%, respectively, P = 0.02), but after multivariate regression analysis, the performance of an amniocentesis was not an independent contributor to PTB < 28 weeks ( P = 0.90). The performance of an amniocentesis prior to cerclage did not independently contribute to PTB less than 28 weeks.