Meconium in the amniotic fluid and fetal acid-base status

Of 323 pregnancies with meconium-stained amniotic fluid at 36-42 weeks' gestation, 68 (21%) had a pH less than 7.20 in umbilical arterial blood, 21 (7%) had a pH less than 7.15, and only three newborns (0.9%) had true metabolic acidemia. At birth, of the 74 newborns with normal electronic fetal heart rate (FHR) tracings, eight (11%) had an umbilical arterial pH less than 7.20. There was a significantly higher frequency of acidemia (defined as pH less than 7.20) in newborns with both baseline and periodic FHR abnormalities. Although there was a significant difference (P less than .05) in the frequency of meconium found below the cords in these neonates with an umbilical artery pH less than 7.20 compared with those with values exceeding 7.20, there was no significant difference in the frequency of clinical meconium aspiration syndrome. We conclude that meconium-stained amniotic fluid correlates poorly with infant condition at birth as reflected by umbilical cord acid-base measurements.