Parity and the risk of developing rheumatoid arthritis: results from the Swedish Epidemiological Investigation of Rheumatoid Arthritis study

Objective To study the impact of parity history on the risk of antibodies to citrullinated peptide antigens (ACPA) positive and ACPA-negative rheumatoid arthritis (RA), in different age-groups. Method Data from a population-based case-control study of female incident RA cases were analysed (2035 cases and 2911 controls, aged 18–70 years ). Parity history was assessed through a questionnaire. Parous women were compared with nulliparous, by calculating odds ratios (ORs) with 95% confidence interval (CI). Results Parity was associated with an increased risk of ACPA-negative RA in women aged 18–44 years (OR=2.1, 95% CI 1.4 to 3.2), but not in those aged 45–70 years (OR=0.9, 95% CI 0.7 to 1.3). Among young women, an increased risk of ACPA-negative RA was found in those who gave birth during the year of symptom onset (OR=2.6, 95% CI 1.4 to 4.8) and who were at a young age at first birth (