We investigated obstetric outcomes and comorbidities during pregnancy in females with preexisting lupus nephritis (LN) and identified predictors for renal flare. In cases of renal flare during pregnancy, we assessed the long-term post-delivery renal outcome. We performed a retrospective analysis of 183 systemic lupus erythematosus (SLE) pregnancies including blood chemistry, urinalysis, urinary protein, and disease activity recorded at prepregnancy, during pregnancy, and at one month, six months, and one year post-delivery. Pregnancies with preexisting LN had a greater frequency of adverse obstetric outcomes and maternal comorbidity. Renal flares occurred in 50.7% of pregnancies with preexisting LN, 89.2% of which were reactivations. Renal flare among pregnancies with SLE was predicted based on preexisting lupus nephritis (OR 17.73; 95% CI, 5.770–54.484), an active disease prior to pregnancy (OR 2.743; 95% CI, 1.074–7.004), and prepregnancy eGFR < 90 ml/min/1.73 m² (OR 11.151; 95% CI, 3.292–37.768). Persistent LN one year after delivery was observed in 33.3% of pregnancies. The median follow-up time after delivery was 5.9 (3.1–9.7) years and chronic kidney disease (CKD) occurred in 21.4% of pregnancies with renal flare. In patients with renal flare, failing to achieve a ≥ 50% reduction in urine protein levels within six months, longer total duration of renal flare, and acute kidney injury at renal flare was associated with CKD development. Females with preexisting LN should achieve remission before pregnancy. When patients experience renal flares during pregnancy, it is important to reduce the proteinuria level by >50% within six months and to achieve early remission for excellent long-term renal outcomes.