This study sought to assess the relationship between intertwin delivery interval (ITDI) and metabolic acidosis in the second twin at birth using a retrospective twin delivery cohort from a tertiary-level teaching hospital. Twin births were identified from an obstetrical database during a 10-year period from 1994 to 2004. Mean arterial cord pH and base deficit among different ITDIs were compared by analysis of variance. Logistic regression models were used to estimate effects of ITDI on metabolic acidosis. The incidence of metabolic acidosis in the second twin was defined as pH < 7.0, and base deficit was defined as >or= 12 mmol/L at birth. After excluding those pregnancies with both twins delivered by cesarean section, birthweight less than 750 g of either twin, antepartum death of either twin, or second twins with missing cord arterial pH, we had 310 twin pairs left for final analysis. Mean pH was significantly lower and base deficit significantly higher for second twin after ITDI exceeded 60 minutes. The incidence of metabolic acidosis increased with increasing ITDI (chi2 test for linear trend, P = 0.02) and the risk of metabolic acidosis (odds ratio, 22.6; 95% confidence interval, 2.5 to 494.1) was increased in the second twins with ITDI longer than 60 minutes compared with those with ITDI less than 15 minutes. The incidence of metabolic acidosis increases with increasing ITDI and there is a statistically significant increased risk of neonatal acidosis after longer than 60 minutes compared with less than 15 minutes of ITDI.