Influence of maternal body mass index on accuracy and reliability of external fetal monitoring techniques

Objective To evaluate the performance of external electronic fetal heart rate and uterine contraction monitoring according to maternal body mass index. Design Secondary analysis of prospective equivalence study. Setting Three US urban teaching hospitals. Sample Seventy-four parturients with a normal term pregnancy. Methods The parent study assessed performance of two methods of external fetal heart rate monitoring (abdominal fetal electrocardiogram and Doppler ultrasound) and of uterine contraction monitoring (electrohystero-graphy and tocodynamometry) compared with internal monitoring with fetal scalp electrode and intrauterine pressure transducer. Reliability of external techniques was assessed by the success rate and positive percent agreement with internal methods. Bland-Altman analysis determined accuracy. We analyzed data from that study according to maternal body mass index. Main outcome measures We assessed the relationship between body mass index and monitor performance with linear regression, using body mass index as the independent variable and measures of reliability and accuracy as dependent variables. Results There was no significant association between maternal body mass index and any measure of reliability or accuracy for abdominal fetal electrocardiogram. By contrast, the overall positive percent agreement for Doppler ultrasound declined (p=0.042), and the root mean square error from the Bland-Altman analysis increased in the first stage (p=0.029) with increasing body mass index. Uterine contraction recordings from electrohysterography and tocodynamometry showed no significant deterioration related to maternal body mass index. Conclusions Accuracy and reliability of fetal heart rate monitoring using abdominal fetal electrocardiogram was unaffected by maternal obesity, whereas performance of ultrasound degraded directly with maternal size. Both electrohysterography and tocodynamometry were unperturbed by obesity.