Major physiologic adaptations to the demands of pregnancy include volume expansion and vasodilation, heightened renal plasma flow, and glomerular hyperfiltration.1 The accompanying decline in serum creatinine concentration (Scr) occurs by about 16 weeks’ gestation.2 A previous study attempted to define the effect of decreases in midpregnancy kidney function on pregnancy outcomes but had few participants, failed to use a validated reference standard for kidney function during pregnancy, and was confounded by sampling bias.