Prospective study offetal heart rateand rhythm patterns

SUMMARY Thefetal heart beat, detected byultrasound, was recorded forfive minutes from934 antenatal women-266at30to35weeks' gestation and718at36to41weeks. Episodes ofbradycardia 180/minute ranging from30-90seconds' duration o curred in1 (0.4%)oftheformer andin4(0.6%) ofthelatter. Premature beats werenotdetected between 30and35weeks, butoccurred in12(17%)of720at36to41weeks' gestation. 50subjects weremonitored atboth30to35weeksand36to41weeks' gestation andinone woman premature beats werepresent inthelatter butnotintheformer recording. Theincidence (1.7%)ofpremature beats inthefetus at36to41weeks' gestation was similar tothatinthehealthy neonate(0.8%). Recordings ofarrhythmias or ratesoutside therange100-180/minute were replayed through a standard antepartum fetal heart ratemonitor. Themonitor failed todetect premature beats, 2of4 episodes oftachycardia >180/minute, and9of12episodes ofbradycardia <100/minute, stressing its unreliability fordetecting rapidly changing rates.