Estimation of fetal weight by ultrasound measurement. I. Development of a new formula.

A formula for the estimation of fetal weight combining real-time ultrasonic measurement of the fetal biparietal diameter (BPD) and the abdominal transverse diameter (ATD) has been developed. The computer-aided regression analysis was based on measurements of 396 unselected fetuses with birth weight range 680-5080 g. The measurements were made within 4 days of delivery. A main purpose was to construct a formula which could be used for prediction of fetal weight within the range 1000-5000 g and where special emphasis was placed on accuracy in the low and high weight groups. A stratified sample of 150 fetuses was therefore selected at random from the whole group. To obtain an optimum curve fitting between the explanatory variables and birth weight, various regressions of quadratic and cubic degree, regressions of exponential functions and power functions were made. The best fit was provided by a power function of the variables. In the second part of this study, this formula will be tested on a clinical material.