To establish the reliability of fetal magnetocardiography as a method of measuring the time intervals of the fetal heart during the antenatal period. A prospective study. Wellcome Biomagnetism Unit, Southern General Hospital. One hundred and six low risk pregnant women at 20 to 42 weeks gestation. Success in obtaining QRS complexes, P waves and T waves. Correlation of time intervals with fetal outcome. The technique was acceptable to pregnant women. A QRS complex was successfully demonstrated in 68 (67%) of the unaveraged traces. Using off-line averaging techniques on these 68 cases, P waves were obtained in 75% and T waves in 72%. Although good quality traces were obtained throughout the range of gestational ages, in general it was more difficult below 28 weeks. QRS duration (R2 = 7%, P = 0.02) demonstrated a positive linear correlation with increasing gestation. Of the 35 (51%) cases with umbilical vein pH analysis available, only one result was less than 7.2. No significant relation was found between measurements of the fetal waveforms and the pH results. The technique of fetal magnetocardiography provides a significant advance in the technological field for the demonstration of QRS complexes and the full PQRST waveforms in gestations from 20 weeks onwards. With further technical improvements the clinical impact of this technique can be assessed more fully.