What is the minimum number of heart rates necessary to evaluate fetal conditions 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 distribution matrices previously derived from 10,934,604 FHRs of 743 uncomplicated fetuses. These matrices were made at nine consecutive 2-week intervals between 23 and 40 weeks' gestation, from which samples were taken after assigning random numbers to FHRs in sequence. As a variable, the difference rate (%) between the sample probability distribution matrix and its corresponding age-group probability distribution matrix was calculated. Scattergrams of difference rates vs. a given number of FHR samplings were analyzed using piecewise linear regression. One critical point per age-group emerged, ranging between 9000 and 10,000 beats, for all age-groups. A linear decrease in the difference rate was noted with a step-by-step increase in random sampling size of FHRs until reaching the critical point, beyond which the difference rate remained constant between 25-33%. The critical points indicate that the minimum number of FHRs for assessment of the fetus at 23-40 weeks' gestation is almost the same, between 9000 and 10,000, with 67-75% baseline variability (so called beat-to-beat variability) and 25-33% long-term variability regardless of advance in gestation.
Maternity and Perinatal Care Unit, Kyushu University Hospital, Fukuoka, Japan.
T
T
Koyanagi
T
T
Yoshizato
T
T
Takashima
S
S
Satoh
T
T
Suzuki
H
H
Nakano
D005069:Evaluation Studies as Topic; D005260:Female; D005865:Gestational Age; D006340:Heart Rate, Fetal / Q000502:physiology*; D006801:Humans; D016014:Linear Models; D011247:Pregnancy