Assessment of the accuracy of multiple sonographic fetal weight estimation formulas: a 10-year experience from a single center.

OBJECTIVES

The primary aim of this study was to compare the accuracy of sonographic fetal weight estimation models. The secondary aim was to define the most accurate time (4-7 or 3 days before delivery) for evaluating fetal weight.

METHODS

In this retrospective cohort study, a total of 12,798 sonographic fetal weight estimations were analyzed, of which 9459 were performed within 3 days of delivery and 3339 within 4 to 7 days. The cohort included all singleton pregnancies recorded at a single medical center from January 2000 to December 2010, with 24 weeks' gestation minimum. Predicted birth weights were calculated according to 23 sonographic fetal weight estimation models; in total, 294,354 sonographic weight estimations were evaluated and compared to the actual birth weights.

RESULTS

The accuracy of the models in predicting birth weight differed considerably. The most accurate models used 3 or more fetal measurements followed by models using abdominal circumference only. The models developed by Sabbagha et al (Am J Obstet Gynecol 1989; 160:854-862) proved most accurate, with a mean percent error of -0.2% and greater than 92% of estimates within 15% of birth weight (P < .05). Nineteen sonographic fetal weight estimation models (82.6%) better predicted fetal weight at 4 to 7 days before delivery (P < .001). Twenty-two (95%) of the models were less accurate at the extreme ends of fetal weight.

CONCLUSIONS

Different formulas for fetal weight estimation vary greatly; we recommend that each center should evaluate the most accurate formula according to its attending population. Estimation of fetal weight performed 4 to 7 days before delivery using most models was more accurate than estimations performed 3 days before delivery.