Background: Nutritional exposures during in utero development may have long-lasting consequences for postnatal renal health. Animal studies suggest that specifically maternal dietary protein intake during pregnancy influences childhood kidney function. Objective: We examined the associations of total, animal, and vegetable maternal protein intake during pregnancy with kidney volume and function in school-aged children. Design: This study was conducted in 3650 pregnant women and their children who were participating in a population-based cohort study from early life onward. First-trimester energy-adjusted maternal protein intake was assessed with a food-frequency questionnaire. At the child's age of 6 y, we assessed kidney volume, estimated glomerular filtration rate (eGFR) using serum creatinine and cystatin C concentrations, and microalbuminuria using urine albumin:creatinine ratios. Results: First-trimester maternal total protein intake was associated with a higher childhood creatinine-based eGFR (difference: 0.06 mL x min(-1) x 1.73 m(-2); 95% CI: 0.01, 0.12 mL . min(-1) . 1.73 m(-2) per gram of protein intake). This association was mainly driven by vegetable protein intake (0.22 mL x min(-1) x 1.73 m(-2); 95% CI: 0.10, 0.35 mL . min(-1) . 1.73 m(-2) per gram of vegetable protein intake). These associations were not explained by protein intake in early childhood. First-trimester maternal protein intake was not significantly associated with childhood kidney volume, cystatin C-based eGFR, or the risk of microalbuminuria. Conclusions: Our findings suggest that higher total and vegetable, but not animal, maternal protein intake during the first trimester of pregnancy is associated with a higher eGFR in childhood. Further follow-up studies are needed to investigate whether maternal protein intake in early pregnancy also affects the risk of kidney diseases in later life.