**2.5.2 Window minimum length and ECG segmentation**

The duration of the fetal cardiac cycle varies from 250 msec to 500 msec for a range of fetal heart rate between 240 bpm and 120 bpm. The fetal QRS-complex itself occupies between 50 msec and 70 msec. The fetal heartbeat is detected in a flag window of length 250 msec. This window length serves two criteria; (i) it is the minimum length yielding an acceptable upper threshold of both the deterministic and stochastic noise types inherent in the higher-order statistics of the ECG signals encountered, and (ii) this window length allows the detection of one, two, three, or four fetal heartbeats (FHBs) within one maternal transabdominal cardiac cycle. For example, for maternal heartbeat of 60 bpm, the R-wave-to-R-wave = 1000 msec, and four segments x 250 msec = one maternal cardiac cycle = possible four fetal cardiac cycles.
