Objectives: To investigate the importance of birth characteristics and early life infections on the risk of later rheumatoid arthritis (RA) and juvenile idiopathic arthritis (JIA).
Methods: We performed a nationwide register-based case-control study based on prospectively recorded data on individuals born 1973 or later. Using the Swedish Inpatient Register and the Early Arthritis Register, we identified cases with RA aged 16 or above (n=333) and JIA (n=3334), respectively. From the Swedish Medical Birth Register, we randomly selected four controls per case, matched by sex, year, and delivery-unit. Through linkage to the Medical Birth Register and to the Swedish Inpatient Register information on maternal-, pregnancy- and birth-characteristics, and infections during first year of life, was identified. We calculated univariate and multivariate odds ratios (OR) using conditional logistic regression.
Results: Overall, infections during first year of life were associated with increased risks for sero-negative (OR =2.6, 95% CI 1.0-7.0) but not sero-positive (OR=1.2) RA and for JIA (OR =1.9, 95% CI 1.7-2.1). Low birth weight (OR=0.7) and being small for gestational age (OR=0.5), were associated with reduced risks of RA of borderline statistical significance. Preterm birth (gestational age ¡Ü258 days) was associated with a non-significantly decreased risk of RA (OR=0.6). Large for gestational age (OR=1.6) and having more than 3 older siblings (OR=1.4) were non-significantly associated with risk of RA.
Conclusion: Infections during first year of life, and possibly also factors related to fetal growth and timing of birth, may be important in the etiologies of adult RA and JIA.