OBJECTIVE: To assess the utility of umbilical cord venous blood gas measures for prediction of umbilical artery pH and base deficit acidemia. METHODS: A retrospective cohort study was conducted of all singletons with valid paired arterial and venous cord gas samples delivered at our institution from January 2006 to March 2012. Fetal acidemia was defined primarily as cord arterial blood gas pH less than 7.0. We also evaluated prediction of acidemia, defined as an arterial base deficit 12 mmol/L or greater. Logistic regression was performed to estimate probabilities of fetal arterial pH and base deficit acidemia given venous blood gas pH or base deficit. Receiver operating characteristic curves were derived to determine predictive ability. Venous blood gas pH and base deficit cutoffs associated with 1% or less, 5%, 10%, and 50% probability of fetal acidemia were identified. RESULTS: Of 23,506 births, 11,455 (49%) met criteria for inclusion. The frequency of arterial blood gas pH less than 7.0 was 127 (1.1%); arterial blood gas base deficit 12 mmol/L or greater was 245 (2.1%). Venous blood gas pH (area under the curve [AUC] 0.949, 95% confidence interval [CI] 0.920-0.979; P<.001) and base deficit (AUC 0.969, 95% CI 0.954-0.983; P<.001) were predictors of acidemia based on arterial blood gas pH and base deficit, respectively. Venous blood gas pH cutoffs associated with 1% or less, 5%, or 10% probabilities of arterial blood gas pH less than 7.0 were 7.23, 7.17, and 7.14, respectively. Venous blood gas base deficit values associated with similar probabilities for base deficit 12 mmol/L or greater were 6.3 or less, 8.2 or less, and 9.0 mmol/L or less. For prediction of arterial blood gas pH, adjusting venous blood gas pH for base deficit increased the AUC (0.961, 95% CI 0.938-0.984). Prediction of arterial blood gas base deficit by venous blood gas base deficit was unchanged by adjustment for pH (AUC 0.969, 95% CI 0.955-0.984). CONCLUSION: We demonstrate that venous blood gas parameters are powerful predictors of arterial blood gas pH and base deficit and can be used to predict the likelihood of fetal acidemia when the cord arterial blood gas is not available.