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To estimate whether the reported increase in obstetric acute renal failure in the United States was attributable to a concurrent rise in postpartum hemorrhage, a change in other risk factors, or changes in the diagnosis of acute renal failure.We conducted a retrospective cohort study to quantify temporal trends in acute renal failure among 10,969,263 hospital deliveries in the United States from 1999 to 2011 using data from the Nationwide Inpatient Sample. Dialysis and maternal death were used as markers of severe acute renal failure. Logistic regression was used to determine whether changes in risk factors explained the temporal rise in acute renal failure.Obstetric acute renal failure rose from 2.4 to 6.3 per 10,000 deliveries between 1999–2001 and 2010–2011, a 10% yearly increase (95% confidence interval [CI] 8–11%) and adjustment for risk factors only partially explained the rise (adjusted yearly increase 6%, 95% CI 5–8%). The overall severity of acute renal failure decreased, although acute renal failure with dialysis rose from 0.27 to 0.36 per 10,000 deliveries (P for trend=.01), and acute renal failure associated with maternal death increased from 0.13 to 0.23 per 10,000 deliveries (P for trend=.01). The temporal increase in acute renal failure with dialysis was abolished by adjustment for chronic kidney disease and chronic hypertension.Changes in diagnosis or ascertainment have led to substantial increases in obstetric acute renal failure, whereas increases in chronic kidney disease and chronic hypertension among pregnant women have resulted in modest but significant increases in acute renal failure with dialysis.