Ontogenesis of ultradian rhythm in the human fetus, observed through the alternation of eye movement and no eye movement periods

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Abstract
To evaluate the ontogenesis of ultradian rhythm of alternating eye movement (EM) and no eye movement (NEM) period durations, a cross-sectional study was conducted on 240 human fetuses from 25 to 41 weeks of gestation, using real-time ultrasound to observe eye movements. Two critical points for the duration of the EM period were found at 29-30 and 37-38 weeks of gestation, whereas for the duration of the NEM period, two critical points were again found, at 31-32 and 37-38 weeks of gestation. However, the mean duration of the EM period was constant from 25-26 to 29-30 weeks of gestation, and then increased from 29-30 to 37-38 weeks of gestation. The mean duration of the EM period was also almost constant from 25-26 to 31-32 weeks of gestation, and then increased from 31-32 to 37-38 weeks of gestation. Durations of both the EM and NEM period become stable with diminished variance after the same gestational age of 37-38 weeks. These findings could imply that two separate mechanisms exist, one for maintaining the duration of the EM period and another for maintaining the duration of the NEM period. Both switching and maintaining mechanisms responsible for ultradian rhythm of the EM and NEM periods have matured at 37-38 weeks of gestation.