Umbilical cord gases are often use to assess the impact of labor and delivery on the fetus. However, no large series exists which reflects contemporary obstetric practice or analyses blood gas ranges by route of delivery. Baseline, pre-labor acid-base status in the human fetus is also poorly defined, rendering assessment of blood gas changes during labor difficult.
To define normal umbilical cord gas and lactate values stratified by mode of delivery in a large contemporary series in which universal cord gas collection was dictated by protocol.
Retrospective cohort study. We analyzed umbilical cord gas and lactate data from an unselected population of infants born between March 2012 and April 2022 at a large teaching hospital. These values were then analyzed by mode of delivery and, for cesarean deliveries, by indication for cesarean and type of anesthesia. Cord gas values from infants delivered by elective cesarean under general anesthesia without labor were considered representative of baseline, pre-labor values.
Data was available for 45,475 infants. Median arterial pH values and interquartile ranges for vaginal births, elective cesarean births without labor and cesarean births performed for fetal heart rate concerns were 7.27 (0.09), 7.27 (0.06) and 7.25 (0.09), respectively. Arterial lactate values for these same 3 groups were 4.1 (2.5), 2.5 (1.2) and 4.0 (2.8) mmoles/l. Due to the very large sample size, most comparisons yielded differences which were statistically significant, but clinically irrelevant. Of all infants, 14% had an arterial pH < 7.20; a pH value of 7.1 represents 2 standard deviations from the mean.
This large, population-based study of cord gas/lactate levels in an unselected population stratified by delivery mode represents a previously unavailable benchmark for evaluation of umbilical cord gases. Arterial cord pH values for infants delivered by elective caesarean without labor (median pH 7.28) reflect a lower pre-labor fetal pH baseline than that previously assumed. This finding, coupled with our determination of a 2 standard deviation lower pH limit of 7.1 rather than the historic arbitrary pH 7.2 threshold of normal helps explain the poor positive predictive value of electronic fetal heart rate monitoring, a test designed to detect arterial pH levels which have fallen from an assumed baseline near pH 7.4 to an assumed potentially injurious pH level < 7.2. Uncomplicated labor, even when prolonged, does not generally result in a clinically significant cumulative hypoxic stress to the human fetus. These findings, along with our determination of no difference in acid-base status among infants delivered by cesarean for fetal heart rate concerns, help explain the failure of current approaches to labor and delivery management in reducing the rates of neonatal hypoxic-ischemic encephalopathy and cerebral palsy, condition which almost always reflect developmental events rather than the effects of labor on the fetus.

Copyright © 2023. Published by Elsevier Inc.