Disease activity 6 months before pregnancy of patients with systemic lupus erythematosus (SLE) associated with adverse maternal and fetal outcomes is not well studied. The aim of the study was to identify predictors of adverse maternal and fetal outcomes in pregnant SLE patients, based on patients’ background characteristics, clinical and laboratory data 6 months before pregnancy. Of 103 pregnancies, 55 pregnancies in 39 SLE patients were investigated. Clinical and laboratory data were recorded at regular intervals from 6 months before conception to 1 year after delivery. Primary outcomes included the predictors of combined adverse maternal and fetal outcomes. Potential explanatory variables included demographic, clinical and laboratory data 6 months before conception. Using logistic regression, history of nephritis ( p = 0.001, odds ratio [OR] 13.3, 95% confidence interval [CI] 2.7–65.1) and a high SLE Disease Activity Index (SLEDAI) score 6 months before pregnancy ( p = 0.015, OR 1.7, 95% CI 1.1–2.7) were associated with combined adverse maternal outcome, whereas flare during pregnancy ( p = 0.003, OR 29.3, 95% CI 3.1–273.1) predicted combined adverse fetal outcome. The area under the curve for SLEDAI score of combined maternal outcome was 0.73 (95% CI 0.58–0.87). The optimal cut-off point according to the receiver operating characteristic curve was 4, with a sensitivity of 64% and a specificity of 75%. In conclusion, a history of nephritis or a SLEDAI score of 4 or more in SLE patients 6 months before conception predicts adverse maternal outcomes, while disease flare during pregnancy predicts adverse fetal outcomes. Pregnancies should be delayed until the disease has been in remission for 6 months.