Rhythm abnormalities of the fetus
he myocardium begins to contract rhythmically by 3 weeks post-conception as a consequence of the activity of spontaneously depolarising myocardial pacemaker cells of the embryonic heart, and its maturation continues even into the postnatal period. While the exact timing of onset of the atrioventricular (AV) electromechanical relationship remains speculation in humans, by 6 weeks post-conception AV synchrony can be demonstrated using standard Doppler techniques. By 5-6 weeks the normal mean fetal heart rate is 110 beats/min (bpm). With further growth and maturation of the conduction system, including definition of the sinoatrial node as the primary cardiac pacemaker with its highest intrinsic rate of spontaneous depolarisation, there is a subsequent increase in the rate to 170 bpm by 9-10 weeks. The rise in heart rate is followed by a decrease to 150 bpm by 14 weeks, likely as a consequence of increasing parasympathetic control and improved myocardial contractility. By 20 weeks the average (SD) fetal heart rate is 140 (20) bpm with a gradual decrease to 130 (20) bpm by term. In the healthy fetus the heart rate is regular, usually remains between 110 and 180 bpm, and has a beat-to-beat variation of 5-15 bpm.