The effect of labor on the normal values of umbilical blood acid-base status

Although several investigators have attempted to define the normal values of umbilical cord blood pH and gases, there is considerable controversy about the optimal cutoff values to diagnosis intrauterine asphyxia. A possible reason for this might be that several studies have included data from fetuses born after different duration of labor.

To determine the effect of labor and the duration of second stage on labor on umbilical arterial acid-base status at birth in healthy term infants.

Umbilical artery acid-base status was determined in patients (n = 356) who met the following criteria: 1) singleton term pregnancy with no significant medical, obstetric, or neonatal complications; 2) vertex presentation if delivered vaginally; 3) neither regional nor general anesthesia applied if delivered vaginally; 4) no use of oxytocin; 5) normal fetal heart rate patterns; 6) clear amniotic fluid; 7) Apgar scores at 1 and 5 min > or = 7; 8) appropriate fetal weight for gestational age. Patients were divided into three groups. A-patients delivered by cesarean section (CS) in the absence of labor (n = 135); B-patients delivered by CS during first stage of labor (n = 62); C-patients with vaginal birth (n = 159). A Kruskal-Wallis ANOVA with post-hoc procedures and stepwise multiple regression analysis were performed.

1) There were significant differences in cord arterial acid-base values between study groups (pH: no labor CS, 7.27 +/- 0.05 vs labor CS, 7.26 +/- 0.05 vs vaginal birth, 7.24 +/- 0.07, p < 0.00001). 2) There was a significant fall in cord arterial pH and bicarbonate concentrations with increased duration of second stage of labor in newborns born vaginally (pH: duration of second stage, 1-30 min, 7.25 +/- 0.07 vs 31-60 min, 7.22 +/- 0.06 vs > 61 min, 7.21 +/- 0.07, p < 0.05). 3) Analysis of confounding variables which could influence the cord pH such as parity, use of vacuum, gestational age, maternal age, and birth weight by stepwise multiple regression analysis indicated that only the duration of second stage of labor pain had a significant relationship with cord blood pH.

There is a significant fall in umbilical artery pH and bicarbonate with the presence of labor and increased duration of second stage of labor in healthy term neonates. This should be taken into consideration in evaluating neonatal well-being by cord blood pH and acid-base measurements.