In order to reveal whether or not rhythmic changes exist in fetal stomach movement (FSM), in utero FSM was assessed in three fetuses between 27 and 33 weeks' gestation with congenital duodenal obstruction. A total of four observations, one each at 27, 29, 31 and 33 weeks' gestation, was obtained. The longitudinal transection of each fetal stomach was continuously observed for 60 min using real-time ultrasound. The configuration and the area of the stomach were analyzed for each 15-s epoch. The complexity of the stomach configuration was quantified and defined as stomach complexity. For each case, the chronological changes of the stomach complexity were analyzed using the least median square of regression. The correlation between changes of the stomach complexity and the area of the stomach was analyzed using the cross-correlation method. (1) For gestational ages of 27, 29, 31 and 33 weeks, the 240 sequential measurements of the stomach complexity were significantly stratified into outlying and non-outlying points. The outlying points were 13.3% (32/240), 30.8% (74/240), 32.9% (79/240) and 36.3% (87/240) of the total observation points, respectively. (2) The percentages in which outlying points lasted 3 min (12 points) or more were 0% (0/240), 5.0% (12/240), 28.3% (68/240) and 30.4% (73/240) of the total observation points, respectively. (3) For each gestational age, no significant time series correlation was found between the stomach complexity and the area of the stomach. These findings suggest that: (1) two different conditions emerge in the FSM, at the latest at 27 weeks' gestation, and begin manifesting from 29 weeks' gestation onwards. (2) These chronological changes cluster into 'active' and 'quiet' phases from 31 weeks' gestation onwards. (3) FSMs are not related to the changes in the stomach area throughout the observation periods. The underlying mechanism of this rhythmicity may represent the development of ultradian rhythm of the stomach movement, generated by the central nervous system.