Active and quiet periods in the preterm and term fetus.

Heart rate variability and fetal movement were used to classify active and quiet fetal periods. Fetuses at 28 to 30 weeks and at 38 to 40 weeks' gestation were compared. A period of fetal movement with increased variability of heart beat was classified as active and one with the absence of movement and diminished fetal heart beat was classified as quiet. The results demonstrated significant differences both in the number of active-quiet cycles per hour and in the length of the active periods. These findings suggest that analogs of fetal behavioral states may be present before birth and have cyclic patterns, and that the time spent in each fetal period is longer in older fetuses.