Background and Significance: Cesarean delivery rates increased by more than 50% between 1996 and 2011 in the United States. The large increase in rates for the procedure was generally not associated with significant improvements in obstetric outcomes, raising concern about quality and prompting recommendations for prevention. Primary cesareans provide the best opportunity to reduce overall cesarean rates, and the group of first-time mothers considered low-risk for cesarean (known as nulliparous, term, singleton, vertex, NTSV) constitutes the focus of prevention efforts. Studies increasingly report racial and ethnic differences in NTSV cesareans, which remain after controlling for health factors. However, the reasons for these disparities and whether or not they can be mitigated are issues that are not well known. The objective of this investigation was to examine factors that modify the association between race, ethnicity and NTSV cesarean deliveries in Florida. Our overall aim was to improve understanding of drivers of racial and ethnic disparities in cesareans in order to inform efforts to reduce disparities. Methods: We conducted a population-based retrospective cohort study of 145,117 NTSV deliveries in labor, using Florida’s linked birth certificate and maternal hospital discharge records for the period of 2012 to 2014. The study was restricted to births in routine delivery hospitals to five racial and ethnic groups: non-Hispanic whites and blacks (including Haitians), Cubans, Puerto Ricans, and Mexicans. Two contrasting approaches were employed in the analysis. First, generalized linear mixed modelling was used to examine, quantify and describe effect modification of the race/ethnicity–association by cesarean risk factors. Non-Hispanic whites were the reference group for comparison. Second, classification tree modeling (chi-Squared Automatic Interaction Detection, CHAID) was used to identify cesarean risk factor combinations that define distinct subgroups with high and low rates of NTSV cesarean among the different racial and ethnic groups in the study population. Risk factors examined included individual socioeconomic, medical and health service-related factors, hospital factors, and a maternal neighborhood index of deprivation/affluence. Results: Non-Hispanic whites were the largest racial/ethnic group in the study population (57.6%), followed by non-Hispanic blacks (23%), Cubans (8.1%), Puerto Ricans (6.8%) and finally Mexicans (4.5%). All four minority groups experienced a higher risk of cesarean relative to non-Hispanic whites after adjusting for significant risk factors, with Cubans having the highest adjusted risk ratio (RR, 1.27) followed by non-Hispanic blacks (RR, 1.18). From the regression-based tests of effect modification, we found positive interactions between race (non-Hispanic black versus white), older gestational age, and labor induction; and negative interactions between ethnicity (Cuban versus non-Hispanic white), presence of medical risk conditions, and labor induction. The adjusted RR of cesarean comparing blacks to whites was 1.04 among spontaneous deliveries at early term (P=.33), but increased to 1.28 (P Conclusions: Our findings on risk factors that modified the association between race, ethnicity and NTSV cesarean delivery and differences in cesarean risk subgroups between racial and ethnic groups suggest that there are potential opportunities to reduce disparities in rates for the procedure in Florida. Whereas racial disparities appear to be related to disparities in health service factors related to cesarean, ethnic disparities appear to persist above and beyond the medical and health service factors included in this investigation. Further research, potentially involving qualitative methods and targeting some of the identified maternal subgroups with high rates of cesarean may help clarify maternal cultural factors, or differences in patient-provider interaction, that may contribute to some of the disparities.