We observed fetal behavior by using three ultrasonic real-time scanners simultaneously in 30 normal pregnant women between 20 and 40 weeks of gestation and analyzed developmental changes in behavioral pattern focussing the attention on the characteristics of REM and non-REM sleep and wakeful state. The frequency of REM significantly increased with gestational age, and its occurrences formed a group in late pregnancy. REM and non-REM periods, which were defined as the phase with uninterrupted appearance of rapid eye movements and the phase without them over 3 min, respectively, became clearly distinguishable between 28 and 31 weeks of gestation. The correlation between the occurrence of rapid eye movements and breathing movements was high after 27 weeks. These results demonstrate the course of the development of sleep-wakefulness cycle in the human fetus.