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Objective: To study perinatal characteristics as risk factors for developing primary Sjogren's syndrome (SS)., Methods: This was a case control study with extraction of information from birth records comprising 32 cases with SS (fulfilling the unified American-European classification criteria) and 159 controls. Cases were selected from a patient register of SS cases in Malmö, Sweden. For each case, 5 controls (living in the same catchment area, matched by date of birth, sex, and delivery unit) from the general population were identified. The relative risks of developing SS were assessed as odds ratios (OR). The primary predictor searched for was birth weight. Secondary predictors were breastfeeding during postpartum hospital stay, paternal occupation, placenta weight, gestational length, diseases during pregnancy, maternal age, parity, and history of miscarriage., Results: Significantly increased OR were observed for high birth weight (>/= 4000 vs 3000-3999 g, OR = 3.8 95% confidence interval, CI: 1.3-11.7) and low maternal age (p < 0.05). Low paternal socioeconomic status (OR = 3.2, 95% CI: 1.0-10.5) and being first-born (OR = 2.5 95% CI: 1.0-5.0) tended to be associated with SS., Conclusions: Our findings suggest that characteristics of the perinatal period may be of etiologic importance in the pathogenesis of SS. Possible mechanisms include modulation of the immune system early in life. It is conceivable that birth weight may be a marker for qualitative and/or quantitative differences in the immune system.