Even though fetal heart rate recordings are widely used to monitor fetal health, both antenatally and in labour, the underlying physiology is not well understood. For example, it is not known with any certainty whether the oscillations seen in fetal heart rate are highly organised, in reflection of underlying ultradian rhythms, or whether they are entirely random and haphazard. In order to answer this question, therefore, we have used mathematical techniques of time series analysis to look for clear evidence of ultradian rhythms in fetal heart rate recordings. We have found that specific short-term ultradian rhythms are indeed present, and that they can be measured objectively in terms of their frequency, amplitude and phase. Such rhythms have cycle lengths of 10 to 90 s and they can persist for long periods of time. They may also disappear and later reappear, locking back into synchrony again with previous oscillations. Individual rhythms may undergo amplitude change, phase shift, and perhaps even frequency shift.