BACKGROUND: Kidney function is not routinely assessed during pregnancy. Several studies have proposed antepartum kidney function, particularly 2nd trimester kidney function, as a potential predictor of adverse pregnancy outcomes. It has been previously established that patients with Systemic Lupus Erythematosus are at increased risk for adverse pregnancy outcomes. The association between 2nd trimester kidney function and adverse pregnancy outcomes has not been evaluated in diverse patient populations, particularly among patients with high obstetrical risk. METHODS: In this observational study of pregnant patients with lupus in North America and Europe from 1995-2017, we used 2nd trimester creatinine and the estimated glomerular filtration rate (eGFR) to model the log odds of preeclampsia, preterm birth, low birthweight, fetal loss, and a composite of those outcomes. We incorporated these measures into a regression setting using fractional polynomials, and we further examined discrete formulations of eGFR. RESULTS: Among 684 pregnancies in patients with lupus, the mean 2nd trimester creatinine was 0.63mg/dL ± SD 0.26 and the median value 0.60 (IQR 0.50-0.70). At least 1 in 3 patients in this combined cohort experienced an adverse outcome. Mixtures of U-shaped and linear relationships between continuous kidney function and the log odds of adverse pregnancy outcomes were observed. Stratifying the cohort by diagnosis of lupus nephritis (active or in remission) or without diagnosis of nephritis, we found differences in the relationship between kidney function and adverse outcomes. CONCLUSIONS: We observed high rates of adverse pregnancy outcomes in our diverse patient population comprised of pregnant patients with lupus with and without lupus nephritis. We identified complex relationships between 2nd trimester kidney function and adverse pregnancy outcomes that differed by the outcome and diagnosis of lupus nephritis.