To what extent does a given heart rate correlate with following heart rates in the developing human fetus?

To quantitatively determine the extent to which a given heart rate correlates with the following heart rate(s) at any gestational age, we studied 181 uncomplicated human fetuses between 23 and 41 weeks gestation. A continuous 90-120 min observation was made for each case using external Doppler-ultrasound cardiotocography. For every individual fetal heart rate dataset, 'probability distribution matrices' were calculated with fetal heart rates (termed FHRs) at 1-beat/min (bpm) intervals, and the beat-to-beat(s) difference (termed DFHR(n): 1 ≤ n ≤ 1000), also at 1-bpm intervals arranged in rows and columns, respectively, with probability in each element. Using 'piecewise linear regression', (1) the difference between the DFHR₁ (n = 1) probability distribution matrix as the 'control' and a given DFHR(n) (n ≥ 2) probability matrix as the 'variable', was analyzed to obtain the statistically critical point(s) (termed 'beat-delay' in beats) for each fetus, and (2) a scattergram of 'beat-delay' vs. gestational age-group, made from all fetuses studied, was analyzed to reveal any critical age(s) in gestation. One statistically significant point was evident at 28-29 weeks gestation. During the period prior to the critical point, a linear decrement in 'beat-delay' with 21 beats (mean) at 23-25 weeks to 11 beats at 28-29 weeks gestation was noted, suggesting that the conduction system-oriented heart beat gradually comes under functional control of the autonomic nervous system as well as under possible regulation by the maturating medulla oblongata. From the critical point, through to term, there was no significant change in 'beat-delay' ranging from 7-11 beats, thereby implying that the fetal heart rate becomes stabilized as a result of the control rendered by the more developed autonomic nervous system function. Whether or not brain function cephalad to the medulla oblongata actually participates in this phenomenon remains to be elucidated.