Changes in diurnal variations in the fetal heart rate baseline with advancing gestational age

In order to longitudinally analyze the diurnal variations in fetal heart rate (FHR) baseline, 24-h FHR recordings were made at 21, 24, 27, 30, 33, and 36 weeks for a normal pregnant woman. Significant diurnal variations were found in all gestational weeks. The amplitude of diurnal variation increased with advancing gestation. The FHR baseline at 00:00–09:00 hours significantly decreased from 24 to 30 weeks, while that at 15:00–21:00 hours decreased after 27 weeks. These findings suggest that the changes in the diurnal variations of FHR baseline with advancing gestation reflect the process of development of the fetal nervous system.