[Pattern of normal fetal heart rate in the third trimester of pregnancy]

It has been shown by previous studies that the mean fetal heart rate (FHR) decreases from the 11th to the 30th week, afterwards the values remain more stable. In the present study we evaluated the FHR pattern throughout the last stages of pregnancy and particularly we estimated the interindividual variability to achieve the maturation of the parasympathetic nervous system in the control of the FHR rythm.

Serial FHR determinations were made from the 28th week until term in 126 physiological pregnancies, submitted to umbilical Doppler velocimetry. The mean FHR was measured on the length of 6 cardiac beats. The FHR patterns of 48 healthy pregnancies, were also analyzed longitudinally.

The transversal data, related to the 126 physiological pregnancies, show a further decrease of the FHR during the last stages of pregnancy: from the 28th to 40th week it drops from the average value of 141 bpm to 133 bpm (r = -0.1469). The longitudinal data show that this decrease occurs in 27% of patients between the 30th and the 33rd week and in 73% of patients between the 34th and the 40th week.

The conclusion is drawn that the physiological decrease of the mean FHR value occurs for each fetus at different gestational ages due to a different balance between the sympathetic and parasympathetic tone; moreover, we find that the interindividual variability in achieving the full maturation of the parasympathetic nervous system is quite wide.