WHAT IS THE MINIMUM NUMBER OF HEART-RATES NECESSARY TO EVALUATE FETALCONDITIONS AT DIFFERENT GESTATIONAL AGES

Our objective was to determine the minimum number of fetal heart rates (FHRs) needed to assess various fetal conditions adequately, focusing on FHR changes in relation to gestational age. We used probability di stribution matrices previously derived from 10 934 604 FHRs of 743 unc omplicated fetuses. These matrices were made at nine consecutive 2-wee k intervals between 23 and 40 weeks' gestation, from which samples wer e taken after assigning random numbers to FHRs in sequence. As a varia ble, the difference rate (%) between the sample probability distributi on matrix and its corresponding age-group probability distribution mat rix was calculated. Scattergrams of difference rates vs. a given numbe r of FHR samplings were analyzed using piecewise linear regression. On e critical point per age-group emerged, ranging between 9000 and 10000 beats, for all age groups. A linear decrease in the difference rate w as noted with a step-by-step increase in random sampling size of FHRs until reaching the critical point, beyond which the difference rate re mained constant between 25-33%. The critical points indicate that the minimum number of FHRs for assessment of the fetus at 23-40 weeks' ges tation is almost the same, between 9000 and 10000, with 67-75% baselin e variability (so called beat-to-beat variability) and 25-33% long-ter m variability regardless of advance in gestation.