HEART-RATE AND FLOW VELOCITY VARIABILITY AS DETERMINED FROM UMBILICALDOPPLER VELOCIMETRY AT 10-20 WEEKS OF GESTATION

1. The aim of this study was to define from umbilical artery flow velo city waveforms absolute peak systolic and time-averaged velocity, feta l heart rate, fetal heart race variability and flow velocity variabili ty, and the relation between fetal heart rate and velocity variables i n early pregnancy. 2. A total of 108 women presenting with a normal pr egnancy from 10 to 20 weeks of gestation consented to participate in a cross-sectional study design. Doppler ultrasound recordings were made from the free-floating loop of the umbilical cord. 3. Umbilical arter y peak systolic and time-averaged velocity increased at 10-20 weeks, w hereas fetal heart rate decreased at 10-15 weeks of gestation and plat eaued thereafter. Umbilical artery peak systolic velocity variability and fetal heart rare variability increased at 10-20 and 15-20 weeks re spectively. 4. The inverse relationship between umbilical artery flow velocity and fetal heart rate at 10-15 weeks of gestation suggests tha t the Frank-Starling mechanism regulates cardiovascular control as ear ly as the late first and early second trimesters of pregnancy. A diffe rent underlying mechanism is suggested for the observed variability pr ofiles in heart rate and umbilical artery peak systolic velocity. It i s speculated that heart rate variability is mediated by maturation of the parasympathetic nervous system, whereas peak systolic velocity var iability reflects the activation of a haemodynamic feedback mechanism.