Natural history determinants of the time constants of the fetal electrocardiogram.

A total of 92 fetuses have been studied during labour using a technique of real-time fetal electrocardiography. This study showed differences in the PR and QRS intervals between male and female infants and also confirmed previous findings of a positive correlation between QRS width and fetal birthweight. The importance of taking natural biological factors into account in the interpretation of the time constants of the FECG is demonstrated.