To investigate the development of sleep rhythm through the analysis of fetal behavior, the alteration of behavior was studied in cases with maternal antiepileptic drug (AED) administration (n = 13) in comparison with normal fetuses (n = 16). The subjects were classified into two groups according to gestational age (32 to 35 weeks and 36 to 40 weeks). The frequency of breathing movement (BM) and the REM and non-REM rhythm modes were affected by AED. The 32 to 35 week group showed an increase in the appearance rate (AP) of REM, a decrease in the duration of REM periods and normal non-REM periods. The 36 to 40 week group showed an increase in AP and prolonged REM periods while both non-REM periods decreased. A decrease in REM density of the REM period and a low BM frequency were distinctive findings in both groups. These results suggest that AED directly affects the fetal central nervous system, which regulates REM and non-REM sleep at a higher level, to alter sleep modes. The differences in the patterns of behavioral modulation of REM and non-REM rhythms in both gestational age groups induced by AED indicates the degree of central nervous system regulation of REM and non-REM rhythms.