Reference ranges for fetal heart rate patterns in normoxaemic nonanaemic fetuses.

Reference ranges for the various components of fetal heart rate (FHR) patterns with gestation were established from the study of 119 fetuses at 20-39 weeks. These fetuses were proved by cordocentesis to be normoxaemic and nonanaemic. The mean baseline FHR decreased, while the baseline variability, both for amplitude and oscillation frequency, increased with gestation. The number and amplitude of accelerations increased, and the frequency and duration of decelerations decreased with gestation. These data indicate that in the interpretation of FHR patterns adjustments according to the gestational age of the fetus should be made.