OBJECTIVES: To study whether oral contraceptive (OC) use or breastfeeding (BF) influence the risk of rheumatoid arthritis (RA), stratifying the cases by presence/absence of anticitrullinated protein antibodies (ACPA), and whether these factors interact with known risk factors in the development of ACPA-positive RA.METHODS: Women aged ?18 years, participants in the population-based case-control Swedish Epidemiological Investigation of RA study (2641 cases/4251 controls), completed an extensive questionnaire regarding OC, BF and potential confounders. We calculated ORs, with 95% CIs, adjusted for age, residential area, smoking and alcohol consumption. Attributable proportion due to interaction (AP) was estimated to evaluate presence of interaction.RESULTS: Compared with never users, ever and past OC users had a decreased risk of ACPA-positive RA (OR=0.84 (95% CI 0.74 to 0.96); OR=0.83 (95% CI 0.73 to 0.95), respectively). No significant associations were found for ACPA-negative RA. Long duration of OC use (>7 years vs never use) decreased the risk of both ACPA-positive (p=0.0037) and ACPA-negative RA (p=0.0356).A history of long BF decreased the risk only of ACPA-positive RA in a dose-dependent manner (p=0.0086), but this trend did not remain after adjustments. A significant interaction was observed between the lack of OC use and smoking (AP=0.28 (95% CI 0.14-0.42)) on the risk of ACPA-positive RA. No interactions were found for BF.CONCLUSIONS: OC decreased the risk of RA, especially ACPA-positive RA, where an interaction with smoking was observed. A long duration of OC use decreased the risk of both disease subsets. We could not confirm an association between BF and a decreased risk of either ACPA-positive or ACPA-negative RA.
Adult Arthritis, Rheumatoid