C3 and lupus nephritis: The multivariate analysis using these variables identified a history of lupus nephritis (OR 6.267, 95% CI 1.750–22.443, P = 0.005) and pre-pregnancy serum C3 level < 85 mg/dl (OR 4.754, 95% CI 1.373–16.461, P = 0.014) as the risk factors for preterm birth (Table 3).