Numerical analysis of the human fetal heart rate: modulation by breathing and movement.

Signal averaging has been used to identify the modulation of fetal heart periods (pulse intervals) by body movements and breathing, identified by a real-time ultrasound scan in normal pregnancies of 37 to 39 weeks. Accelerations were associated with fetal movements and were accompanied by arrest of fetal breathing and a decrease in beat-to-beat variation. They were commonly succeeded by decelerations, with an increase in beat-to-beat variation, within 15 to 20 seconds. In the absence of gross accelerations or decelerations, fetal breathing was associated with a high-frequency modulation of heart periods, measureable as a small increase in beat-to-beat variation. There was clustering of this characteristic pattern around and within fetal breathing episodes, which suggests that it may be a more sensitive index of breathing than visual identification with ultrasound. The conclusions are: (1) the grosser episodic changes in fetal heart rate are associated with the regular repetition of a pattern of accelerations and movements repeated at intervals less than 2 minutes; (2) fetal breathing episodes are independent of these gross changes; (3) beat-to-beat variation alters in a complex fashion and in different directions in association with physiologic changes.