BACKGROUND: Pregnant women with polycystic ovarian syndrome (PCOS) experience a greater rate of adverse obstetrical outcomes compared with non-PCOS women. We examined the prevalence and incidence of cervical insufficiency (CI) in a community cohort of pregnant women with and without PCOS. METHODS: A retrospective cohort study was conducted within a large integrated health care delivery system among non-diabetic PCOS women with second or third trimester delivery during 2002-2005 (singleton or twin gestation). PCOS was defined by Rotterdam criteria. A non-PCOS comparison group matched for delivery year and hospital facility was used to estimate the background rate of CI. Women were designated as having new CI diagnosed in the index pregnancy (based on cervical dilation and/or cervical shortening) and prior CI based on prior diagnosis of CI with prophylactic cerclage placed in the subsequent pregnancy. RESULTS: We identified 999 PCOS women, of whom 29 (2.9%) had CI. There were 18 patients with new CI and 11 with prior CI having prophylactic cerclage placement; four CI patients had twin gestation. In contrast, only five (0.5%) non-PCOS women had CI: two with new CI and three with prior CI. The proportion of newly diagnosed incident CI (1.8 versus 0.2%) or prevalent CI (2.9 versus 0.5%) was significantly greater for PCOS compared with non-PCOS pregnant women (both P < 0.01). Among PCOS women, CI prevalence was particularly high among South Asians (7.8%) and Blacks (17.5%) compared with Whites (1%) and significantly associated with gonadotropin use (including in vitro fertilization). Overall, the PCOS status was associated with an increased odds of prevalent CI pregnancy (adjusted odds ratio 4.8, 95% confidence interval 1.5-15.4), even after adjusting for maternal age, nulliparity, race/ethnicity, body mass index and fertility treatment. CONCLUSION: In this large and ethnically diverse PCOS cohort, we found that CI occurred with a higher than expected frequency in PCOS women, particularly among South Asian and Black women. PCOS women with CI were also more likely to have received gonadotropin therapy. Future studies should examine whether natural and hormone-altered PCOS is a risk factor for CI, the role of race/ethnicity, fertility drugs and consideration for heightened mid-trimester surveillance in higher risk subgroups of pregnant women with PCOS.