Birth weight prediction by three-dimensional ultrasonography - Fractional limb volume

Objective. To introduce fractional limb volume as a new ultrasonographic pa rameter, validate reliability of fractional limb volume measurements, devel op new birth weight prediction models, and examine their practical utility for estimating fetal weight during late pregnancy. Methods. Healthy late-th ird-trimester fetuses were prospectively scanned by two- and three-dimensio nal ultrasonography within 4 days-of delivery. Volume data sets were subseq uently used to extract several standard ultrasonographic measurements. Frac tional limb volumes of the upper arm and-thigh were based on 50% of diaphys eal bone length. Intraclass correlation was used to analyze interobserver a nd intraobserver reliability of fractional limb volume measurements. Severa l weight prediction models were developed by linear regression analysis. Ne w prediction models were prospectively compared with the Hadlock formula in 30 healthy late-third-trimester fetuses. Results. One hundred fetuses were scanned at a mean SD menstrual age of 39.2 +/- 1.2 weeks, Intraclass corre lation indicated a significant degree of interobserver and intraobserver re liability for fractional thigh volume. Fractional thigh volume (r = 0.86), fractional upper arm volume (r = 0.83), abdominal circumference (r = 0.83), and midthigh circumference (r = 0.82) were most highly correlated with bir th weight. The best prediction model (abdominal circumference and fractiona l thigh volume) gave weight estimates that deviated from actual birth weigh t by -0.025% +/- 7.8%. For late-third-trimester fetuses, the Hadlock model yielded errors of 9.0% +/- 9.0%. Prospective testing confirmed superior per formance of the new prediction model, which gave accuracy of 2.3% +/- 6.6% (Hadlock method, 8.4% +/- 8.7%). It correctly predicted 20 of 30 birth weig hts to within 5% of actual weight. By comparison, the Hadlock model predict ed only 6 of 30 birth weights to within 5% of actual weight. Conclusions. A new birth weight prediction model, based on fractional thigh volume and ab dominal circumference, is reliable during the late third trimester. It prov ides a means for including soft tissue evaluation for birth weight predicti on. This rapid technique avoids technical limitations that currently hinder the practical implementation of three-dimensional ultrasonography for esti mating birth weight.