Pre-pregnancy serum complement C3 level is a predictor of preterm birth for pregnancies with systemic lupus erythematosus

Background This study aimed to clarify predictors of preterm birth in pregnancy of women with systemic lupus erythematosus (SLE). We investigated the predictors of preterm birth before pregnancy from the perspective of the importance of preconception care. Methods We analysed fetal outcomes of 108 pregnancies in 74 SLE patients in a retrospective study. We compared pre-pregnancy clinical characteristics and disease activity in these women between the preterm birth and full-term birth groups to select predictive factors for preterm birth before pregnancy. Results Eighty-three of 108 pregnancies resulted in live births, of which 27 (25.0%) were preterm births. Pre-pregnancy serum complement 3 (C3) level was significantly lower in the preterm birth group (77.0 mg/dl) than the full-term birth group (87.5 mg/dl) (P = 0.029). 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. The greater the number of these risk factors, the higher was the preterm birth rate (P = 0.0007). In the case of SLEDAI score ≤ 4, the preterm birth rate was higher in the pre-pregnancy low C3 group (< 85 mg/dl) (42.1%) than in the high C3 group (C3 ≥ 85 mg/dl) (14.7%) (P = 0.018). Conclusion For patients with a history of LN, treatment management focusing on pre-pregnancy serum complement levels is very important.