The rhythmicity of fetal breathing varies with behavioural state in the human fetus.

The rhythmicity of human fetal breathing movements was studied during two different behavioural states (1F and 2F, respectively), using real-time B-scan-directed M-mode ultrasound recordings. The mean breath-to-breath interval durations and the standard deviations (SD), and the standard deviations of the interval differences (SDDSI) were calculated. The mean breath-to-breath interval duration was not significantly shorter during 1F than during 2F. SD and SDDSI, however, showed significantly lower values during 1F, demonstrating that the fetal breathing rhythm is more regular during state 1F than during state 2F epochs. Regular fetal breathing is thus a concomitant of state 1F.