Does use of a sex-specific model improve the accuracy of sonographic weight estimation?

To determine whether the use of a sex-specific sonographic model improves the accuracy of fetal weight estimation.

New regression models (sex-independent and sex-specific) were developed, based on 1708 sonographic weight estimations performed within 3 days prior to delivery. The accuracy of these models was compared to that of several published models including two of the original Hadlock models (which incorporate the biometric indices abdominal circumference (AC), biparietal diameter (BPD), femur diaphysis length (FL) and head circumference (HC) as follows: AC-FL-BPD and AC-FL-HC, designated here as Hadlock I and Hadlock II, respectively), modified versions of the Hadlock I and II models for which coefficients were adjusted to our local cohort, sex-specific versions of the Hadlock I and II models and Schild's model (a previously published sex-specific model).

The unadjusted models of Hadlock and Schild were associated with the highest systematic error (1.6-4.9%; P < 0.001) which was significantly higher for females (2.3-4.9%) compared to males (1.6-2.0%; P < 0.001). Adjustment of model coefficients to the local population decreased the systematic error (-1.4% to 1.5%) and resulted in a systematic error that was of similar magnitude (P = 0.3) but opposite in direction for male and female fetuses. The sex-specific models (adjusted or newly developed) were associated with the lowest systematic error (-0.4 to 0.5%) and were the only models for which the systematic error was similar for male and female fetuses. There were no differences in the systematic error between adjusted sex-specific versions of the Hadlock I and II models and the newly developed sex-specific models (0.0% to 0.4% vs. - 0.4% to 0.5%; P = 0.4). The random error was similar for all models and, for most of the models, was unrelated to fetal sex.

The use of sex-specific models appears to improve the accuracy of fetal weight estimation, principally because the optimal set of model coefficients differs for male and female fetuses. The improved accuracy is mainly the result of a decrease in systematic error, as the random error was not affected by the use of such sex-specific models.