The critical period of non-reassuring fetal heart rate patterns in preterm gestation

Our purpose was to determine the critical period for non-reassuring fetal heart rate (FHR) patterns in preterm gestation in predicting fetal acidosis (umbilical arterial pH <7.1) at birth.

A prospective descriptive study was performed. We reviewed the FHR pattern and umbilical blood gas level measurements, and investigated the correlations between non-reassuring FHR patterns and umbilical arterial pH.

There was a significant difference in the frequency of fetal acidosis between the neonatal death and survival groups (5/13 versus 30/759, P=0.0001). Umbilical pH values in fetuses with persistent late deceleration with loss of variability (7.15+/-0.11, P<0.01) and in those with prolonged deceleration (7.17+/-0.16, P<0.01) were significantly lower than in fetuses with reassuring FHR patterns (7.29+/-0.06). Fetal acidosis also occurred more often in these two groups. The critical periods for late deceleration with loss of variability and for prolonged deceleration were 60 and 30 min, respectively.

These data indicate that a critical period for non-reassuring FHR patterns in preterm gestation exists. Prompt delivery is required within that short critical period.