To investigate the importance of birth characteristics and early life infections on the risk of later rheumatoid arthritis (RA) and juvenile idiopathic arthritis (JIA).
A nationwide register-based case-control study was performed based on prospectively recorded data on individuals born in 1973 or later. Using the Swedish inpatient register and the early arthritis register, cases with RA aged 16 years or above (n = 333) and JIA (n = 3334) were identified. From the Swedish medical birth register (MBR), four controls per case, matched by sex, year and delivery unit were randomly selected. Through linkage to the MBR and to the Swedish inpatient register information on maternal, pregnancy and birth characteristics and infections during the first year of life was identified. Univariate and multivariate odds ratios (OR) were calculated using conditional logistic regression.
Overall, infections during the first year of life were associated with increased risks for seronegative (OR 2.6, 95% CI 1.0 to 7.0) but not seropositive (OR 1.2) RA and for JIA (OR 1.9, 95% CI 1.7 to 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 < or =258 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 three older siblings (OR 1.4) were non-significantly associated with the risk of RA.
Infections during the first year of life, and possibly also factors related to fetal growth and timing of birth, may be important in the aetiologies of adult RA and JIA.