Fetal heart rate variability reveals differential dynamics in the intrauterine development of the sympathetic and parasympathetic branches of the autonomic nervous system.

The aim of this study was to investigate the hypothesis that fetal beat-to-beat heart rate variability (fHRV) displays the different time scales of sympatho-vagal development prior to and after 32 weeks of gestation (wks GA). Ninety-two magnetocardiograms of singletons with normal courses of pregnancy between 24 + 1 and 41 + 6 wks GA were studied. Heart rate patterns were either quiet/non-accelerative (fHRP I) or active/accelerative (fHRP II) and recording quality sufficient for fHRV. The sample was divided into the GA groups <32 wks GA/>32 wks GA. Linear parameters of fHRV were calculated: mean heart rate (mHR), SDNN and RMSSD of normal-to-normal interbeat intervals, power in the low (0.04-0.15 Hz) and high frequency range (0.15-0.4 Hz) and the ratios SDNN/RMSSD and LF/HF as markers for sympatho-vagal balance. fHRP I is characterized by decreasing SDNN/RMSSD, LF/HF and mHR. The decrease is more pronounced <32 wks GA. Beyond that GA SDNN/RMSSD is predominantly determined by RMSSD during fHRP I and by SDNN during fHRP II. In contrast to fHRP I, during fHRP II, mHR is positively correlated to SDNN/RMSSD instead of SDNN >32 wks GA. LF/HF increases in fHRP II during the first half of the third trimester. Non-accelerative fHRP are indicative of parasympathetic dominance >32 wks GA. In contrast, the sympathetic accentuation during accelerative fHRP is displayed in the interrelations between mHR, SDNN and SDNN/RMSSD. Prior to 32 wks GA, fHRV reveals the increasing activity of the respective branches of the autonomic nervous system differentiating the types of fHRP.