BackgroundFolate, vitamin B12 and homocysteine concentrations during pregnancy are important factors for early development and may persistently influence kidney function in the offspring. We examined the associations of folate, vitamin B12, and homocysteine concentrations during pregnancy with kidney outcomes in school-aged children.Study designPopulation-based prospective cohort study from fetal life onwards.Settings & participantsThis study was performed among 4,226 pregnant women and their children.PredictorsFolate, vitamin B12 and homocysteine blood concentrations measured in early pregnancy (median gestational age 13.2 weeks (25th to 75th percentiles 12.2, 14.8) and at birth (cord blood).Outcomes & measurementsAt the median age of 6.0 years (25th to 75th percentiles 5.9, 6.3) we measured combined kidney volume with ultrasound, estimated glomerular filtration rate based on creatinine (eGFRcreat) and cystatin C (eGFRcystC) concentrations and microalbuminuria.ResultsWe observed that higher maternal folate concentrations were associated with larger childhood combined kidney volume, whereas higher maternal vitamin B12 concentrations were associated with higher childhood eGFRcystC (p-values <0.05). These associations were independent of homocysteine concentrations. Higher maternal homocysteine concentrations were associated with smaller combined kidney volume and lower childhood eGFRcystC (p-values <0.05). The association of maternal homocysteine concentrations with childhood eGFRcystC was largely explained by combined kidney volume. Higher cord blood homocysteine concentrations were associated with larger combined kidney volume and lower eGFRcystC (p-values <0.05). Folate, vitamin B12 or homocysteine concentrations were not associated microalbuminuria.LimitationsObservational study, so causality cannot be established.ConclusionOur findings suggest that folate, vitamin B12 and homocysteine concentrations during fetal life are associated with offspring kidney development. However, the effect sizes are small. Further studies are needed to replicate these findings and assess the causality and consequences for kidney health in later life.