Acid-base significance of meconium discovered prior to labor

It has been postulated that meconium is a sign of fetal hypoxia and acidosis. Previous work in this field has been limited. First, previous reports have included only patients with the intrapartum passage of meconium; second, birth asphyxia has been inadequately defined, with reliance only on Apgar scores. There is little information regarding the significance of meconium passage discovered before labor. This paper reports on 40 pregnancies with meconium passage diagnosed before labor. Thirty-three gestations had the diagnosis made at the time of elective repeat cesarean delivery. Seven pregnancies had the diagnosis made at the time of amniocentesis for lung maturity. The seven pregnancies with meconium diagnosed at amniocentesis were promptly delivered by cesarean because of the meconium. Forty uncomplicated pregnancies with clear amniotic fluid at elective cesarean delivery were selected as controls. The umbilicial artery was sampled and an acid-base analysis was performed. Using an umbilical blood artery blood pH cutoff of 7.19 to define acidemia, there were twice as many neonates in the meconium group with respiratory acidemia. This difference did not reach statistical significance. Most importantly, there was not a single infant with metabolic acidemia in either group. All infants had uncomplicated hospital courses. No infant had the meconium aspiration syndrome (Table 1). The authors are reassured by their finding that no neonate had metabolic acidemia and that all had uncomplicated hospital courses. They conclude that meconium discovered before labor is not necessarily a marker of immediate or chronic fetal compromise. Because all gestations were promptly delivered upon the diagnosis of meconium passage, the authors warn against assuming that nonintervention is appropriate when meconium is diagnosed in the antepartum period.