Averaged fMCG waveforms from subject 2(left) and subject 8(right). Subject 8 had 2:1 2° AV block, obtained using matched filtering and 20 s of averaging (top), matched filtering and 200 s of averaging, and splined ICS and 20 s of averaging (bottom). The splined ICS waveform is compatible with the 200 s-average matched filtered waveform, but the 20 s-average matched filtered waveform shows strong residual interference from the maternal T-wave. Although the 200 s-average waveform has higher SNR, notice that the nonconducted P-wave (arrow) overlapping the T-wave is broadened by the longer averaging time, while it remains narrow in the 20 s-average waveforms. Subject 2 had normal sinus rhythm with the fetal and maternal heart rates in a 2:1 ratio.