The effect of preterm birth on umbilical cord blood gases

Apgar scores are used routinely to assess early neonatal status, but are less accurate in the preterm neonate because of developmental immaturity. Attention has been directed to umbilical cord gases as a method of neonatal evaluation. Using a retrospective chart review of all viable preterm births (24-36 weeks’ gestation) between January 1986 and December 1989, we tabulated the umbilical cord gas indices of these infants. Fetuses with lethal congenital anomalies and those with abnormal heart rate tracings on admission were excluded from the data base, leaving 1872 infants. Cord arterial blood gas values were available for analysis in 74.4% of cases and cord venous gas values in 81.8%. The mean (± standard deviation [SD]) arterial and venous umbilical cord blood gas values for the preterm infants were, respectively: pH, 7.26 ± 0.08 and 7.33 ± 0.07; oxygen pressure, 19.0 ± 7.9 and 29.2 ± 9.7 mmHg; carbon dioxide pressure, 53.0 ± 10.0 and 43.4 ± 8.3 mmHg; bicarbonate, 24.0 ± 2.3 and 22.8 ± 2.1 mEq/L; and base excess, –3.2 ± 2.9 and –2.6 ± 2.5 mEq/L. Acidemia was defined statistically as 2 SDs or more below the population mean. The incidence of 5-minute Apgar scores below 7 in the preterm infants was 8.5% and within this group, 17.8% were acidemic (arterial pH 7.10 or lower). More than 82% of neonates with 5-minute Apgar scores less than 7 had normal umbilical cord blood gases. There was no significant difference in umbilical arterial blood gas values between preterm infants and 1924 term deliveries at our institution between 1986–1988. We conclude that there is no significant difference in acid-base status at birth between the preterm and term infant. Because umbilical cord gas indices are an objective means of assessing the immediate status of the newborn, they may be of more value in excluding birth asphyxia than is the Apgar score for preterm infants.