Multivariate analysis identified history of lupus nephritis (odds ratio: 5.734, 95% CI 1.568–21.010, P = 0.008) and low C3 level (< 85 mg/dl) at pre-pregnancy (odds ratio 4.498, 95% CI 1.296–15.616, P = 0.018) as risk factors for preterm birth. This evidence concerns the gene C3 and lupus nephritis.