Fetal heart rate variation described using a probability distribution matrix

We devised a new method using a probability distribution matrix to simultaneously describe variation in the characteristics of fetal heart rate (FHR) and beat-to-beat difference (DFHR) between the present and the immediately following FHR. The FHRs were plotted in columns, DFHRs in rows and probabilities in the corresponding elements of the matrix. The age-related changes of FHR data obtained from 743 fetuses at 23-40 weeks gestation were analysed using a pulsed Doppler cardiotocograph with an autocorrelation system. While keeping an almost symmetrical spread around 0 beats/min of DFHR, three particular probability distribution patterns of FHR versus DFHR emerged with advance in gestation: (i) oval with a monomodal peak from 23/24 to 29/30 weeks, (ii) ellipsoid with a monomodal peak and plateau from 31/32 to 33/34 weeks and (iii) elongated ellipsoid with bimodal peaks from 35/36 weeks of gestation onwards. The probability distribution matrix presented enables one to condense any amount of FHR data into one uniform description. This allows analysis of data, en bloc, achieving a quantitative inter-group comparison, on an equivalent scale.