Increased beat-to-beat variability during uterine contractions: a common association in uncomplicated labor.

A computerized system was used during labor to provide a quantitative and objective analysis of fetal heart rate, beat-to-beat variability, and uterine contractions. Twenty-nine healthy pregnant women at term participated in this study. Each woman was studied for a 40-minute period during the active phase of spontaneous labor. The results indicate that beat-to-beat variability rises from a value of 4.62 +/- 1.11 (mean +/- SD) between contractions to 6.86 +/- 1.53 during contractions. This rise is significant (p less than 0.01). At the same time, changes in fetal heart rate are small, inconsistent, and not significant. We conclude that an increased beat-to-beat variability is commonly associated with uterine contractions in normal fetuses. This increase is probably due to mild hypoxia caused by decreased perfusion of the placenta and to increased vagal tone caused by fetal head compression.