Fetal oxygen saturation measured by pulse oximetry during labour with clear or meconium-stained amniotic fluid

To compare fetal oxygen saturation, scalp pH and arterial cord blood gases in cases of clear or meconium-stained amniotic fluid with and without meconium aspiration (MAS).

Thirty-eight women in labour at term with abnormal fetal heart rate were included. Fetal oxygen saturation was continuously monitored using a Nellcor N-400 fetal pulse oximeter and FS-14 sensor. Fetal scalp blood samples were taken systematically at full dilatation or immediately before cesarean section. Arterial cord blood gases were analysed at birth. Fetal oxygen saturation, scalp pH and neonatal blood gases were compared between fetuses with clear amniotic fluid, meconium-stained amniotic fluid without MAS and meconium stained amniotic fluid with MAS.

Moderate or thick meconium was observed in 13 cases during labour. Three newborns had a meconium aspiration defined as meconium below the vocal cords. No differences were observed in scalp pH, scalp base excess, umbilical arterial blood pH or base excess between groups. On the other hand, fetal oxygen saturation (fSpO2) obtained before birth was significantly lower in cases of MAS when compared to the other groups. This difference appears to be large compared to that which might be attributed to meconium and its direct effect on fetal pulse oximetry readings. Fetal oxygen saturation dropped dramatically in cases with meconium aspiration between the first stage of labour (44.7 +/- 8.0%) and the last measurement before birth (27.0 +/- 8.5%).

Meconium aspiration is more likely to be associated with fetal hypoxemia than with fetal acidosis.