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 (<23) (OR=2.5, 95% CI 1.5 to 4.1). Parity and the postpartum period were not associated with ACPA-positive RA, but older age at first birth was weakly associated with a decreased risk.

Conclusions The increased risk of ACPA-negative RA in parous women of reproductive age seemed to be associated with an increased postpartum risk and with young age at first birth. Further research is needed to explore the biological mechanisms behind our findings.