Accelerations of the human fetal heart rate at 38 to 40 weeks' gestational age.

In order to document the normal structure and pattern of fetal heart rate accelerations near term, we measured fetal heart rate and fetal movements for 24-hour observation intervals in 12 healthy pregnant women at 38 to 40 weeks' gestation. There were 34 accelerations per hour on the average with a mean amplitude of 22.8 bpm and a mean duration of 40.7 seconds. The longest time between successive accelerations was 37 minutes. There was a significant negative correlation between the mean daily maximum amplitude of accelerations and the mean daily fetal heart rate (r = -0.84). Fetal heart rate acceleration patterns suggested a prolonged period of fetal wakefulness during the late evening.