Changes in the fetal heart rate associated with fetal breathing and fetal movement.

Thirty-eight recordings of fetal heart rate and fetal activity were made from 21 normal patients between 36 and 41 weeks gestation. Each recording lasted for an average of 39 minutes. The heart rate was measured from beat to beat using the R-wave of the fetal electrocardiogram as the indicator of each heart-beat. Fetal breathing movements and fetal body movements were detected using either A-mode or B-mode ultrasound systems. The relation of fetal breathing and fetal movement to the fetal heart rate was studied both by cardiotachography and computer analysis of the R--R intervals. Fetal body movements (kicking and rolling) were usually associated with a brief tachycardia, the latter typically occurring every two to three minutes. In 14 recordings, the periods of fetal activity were interrupted by periods of fetal rest with an average duration of 12.5 minutes. Fetal breathing was seen in 26 of the 34 recordings analysed and occurred principally during the periods of fetal activity. Fetal breathing was associated with a significant increase in heart rate variation measured as the standard deviation of the R--R intervals and the mean absolute R--R interval difference. Sometimes a pattern of respiratory sinus arrhythmia was seen.