Measurement of the fetal heart rate during pregnancy by a new electrocardiographic technique.

An external technique for recording the fetal heart rate (FHR) during pregnancy is described. The system is based upon the detection of the fetal electrocardiogram (ECG) from the maternal abdomen and elimination of the maternal QRS complexes by a new system of subtraction. Fetal QRS complexes with an amplitude of 10 microvolts or more were processed to produce heart rate recordings of much higher quality than previously obtained with the phonocardiographic and ultrasonic systems. The proportion of successful recordings was reduced between 27 and 34 weeks of gestation due to the decrease in size of the fetal QRS complex which occurred at this time; this was a major drawback of the ECG system.