The effect of body mass index on three methods of fetal weight estimation

Objective To assess agreement between three methods of estimation of fetal weight and determine the influence of maternal obesity. Design Prospective observational study. Setting A tertiary referral teaching hospital. Population Unselected women attending for induction of labour. Method Maternal, clinical and ultrasonic estimations of fetal weight were made prior to induction of labour in 96 consenting women. The estimations of fetal weight were performed independently by the three methods. Main outcome measure Bland and Altman plots to show limits of agreement, and intraclass correlation coefficients. Results Variable levels of accuracy were obtained for maternal, clinical and ultrasound estimates of fetal weight. Ultrasound estimation of fetal weight performed equally best for women of high and low body mass index (BMI). For women of low BMI, the intraclass correlation coefficient (ICC) was 0.90 (95% CI 0.83-0.94) and 0.87 (95% Cl 0.77-0.93) for women with high BMI. Despite this, the limits of agreement for ultrasound were in the order of -700 to +500 g. Both maternal and clinical estimation of fetal weight under-estimated true fetal weight in women with low BMI and over-estimated the true fetal weight in women with high BMI. The largest observed mean difference was obtained with clinical palpation in both low and high BMI women. Conclusion The accuracy of ultrasound estimation of fetal weight was better than maternal and clinical estimation of fetal weight and was not influenced significantly by maternal BMI.