Blood acid-base balance at birth in neonates from labors with early and late rupture of membranes.

The purpose of this study was to determine whether the early artificial rupture of the amniotic membranes performed to shorten the duration of an otherwise normal labor and delivery might have potentially deleterious effects on the fetus that would be reflected in the neonate. In 38 infants delivered at term, acid-base balances and O2 and CO2 pressures were obtained in umbilical arterial and venous blood at birth, prior to the first inspiration. For the purpose of the study the infants were divided into two groups: group I infants were born after a normal labor in which the amniotic membranes were permitted to rupture spontaneously at full cervical dilatation; group II infants were born after a labor in which the membranes were ruptured artificially when cervical dilatation was 4 to 5 cm. There was no evidence of fetal distress, and all infants were vigorous at birth. The pH of umbilical venous blood was greater in the group with late rupture of the membranes (fiftieth percentile [P50] = 7.36) than in those born after early amniotomy (P50 = 7.30) (p less than 0.01). The pH values of umbilical arterial blood were also higher in the group I infants (P50 = 7.31) than in those born after amniotomy (P50 = 7.25)(p less than 0.025). These differences were also observed in the 19 neonates in whom the cord was not encircled around the neck at the time of birth. The PCO2 in umbilical venous blood was less, and the hemoglobin saturation was greater (P less than 0.05) in group I infants than in those of group II. It is possible that the influence of early amniotomy on fetal pH may be deleterious in infants born after high-risk pregnancies in which the uteroplacental circulation is impaired.