The effect of labor on the acid‐base status of the newborn

Abstract

There has been some controversy regarding the effect that a long labor can have on fetal well-being. This study was undertaken to evaluate the effect of duration of labor both the first and second stage on the acid-base variables in the umbilical artery of the newborn. Bloodsampling was attempted from all infants born at our department between October 1994 and September 1995. Nineteen hundred and forty-one live infants were delivered during the period. Sampling was unsuccessful in 264 cases and after excluding infants with operative delivery, multiple gestations, breech presentation, prematurity, postmaturity and infants small for gestational age 1255 remained singleton, term infants with vertex presentation and non-operative vaginal birth. We found no correlation between duration of the first stage of labor and neither pH nor base excess in umbilical artery blood. There was a significant correlation between duration of the second stage of labor and both the pH and base excess. For vaginal nulliparas we found that pH=7.30-0.016 x second stage duration in hours and base excess=-3.71-0.692 x second stage duration in hours. For women with previous vaginal births pH=7.31-0.029 x second stage duration in hours and base excess=-2.38-1.306 x second stage duration in hours. We find no correlation between duration of the first stage of labor and umbilical artery pH or base excess. We do find a correlation between duration of the second stage of labor and the umbilical artery pH and base excess. However, a prolongation of the second stage with as much as three hours would give an expected lowering of the umbilical artery pH with only 0.05 and of base excess with 2.1 mmol/l for vaginal nulliparas and correspondingly with 0.09 and 3.9 mmol/l in women with previous vaginal births. This effect on pH and base excess is so small that it is hardly clinically relevant and we do not find any support for the belief that a long labor -- in the absence of other risk factors -- is to the disadvantage of the fetus.