Formulas for predicting birthweight are mostly accurate

A review of 35 two-dimensional ultrasonographic algorithms for predicting birthweights found that most of them are relatively accurate, although all the formulas tended to underestimate the size of large fetuses, researchers report in the January issue of Obstetrics and Gynecology.

Marco Scioscia, MD, of the University of Medical Science of Bari in Bari, Italy, and colleagues conducted ultrasonographic examinations of 589 patients about to deliver singleton pregnancies, using 35 different formulas to estimate delivery weight. Of those examined, 441 babies were delivered within 48 hours and were included in the study.

Twenty-nine of the 35 formulas provided an overall mean absolute percentage error less than or equal to 10%. Of those, the percentage of birthweight predictions within 10% (plus or minus) was 69.2%, while the percentage of predictions within 15% (plus or minus) was 86.5%. Twenty formulas showed both good accuracy and low variability. Formulas based on head-abdomen-femur measurements showed the lowest mean absolute percentage error, while formulas based on abdomen and femur measures were most accurate in predicting the weight of fetuses weighing more than 3,500 g.

"Clinically, our findings provide evidence that most formulas have good accuracy at predicting birthweight up to 3,500 g, whereas all estimations beyond that weight have to be carefully considered (clinical evaluation) because all algorithms tend to underestimate large fetuses," the authors conclude.

Scioscia M, Vimercati A, Ceci O, et al. Estimation of birth weight by two-dimensional ultrasonography: a critical appraisal of its accuracy. Obstet Gynecol. 2008;111:57-65.