Longitudinal studies of fetal growth using volume parameters determined with ultrasound

To explore the use of three-dimensional parameters in characterizing fetal growth with ultrasound, the accuracy of volume measurements and the ability to make such measurements during pregnancy have been investigated. Total fetal volumes (TOTV) determined with ultrasound in utero were compared to volume measurements obtained by hydrostatic weighing following induced abortion at 19.4 (+/- 1.8 SD) weeks, menstrual age (MA), and to those calculated from birth weights and density values for term fetuses [39.3 (+/- 0.8 SD) weeks, MA]. This study indicated that ultrasound measurements underestimate the fetal volume [-9.8 (+/- 9.5 SD)% at 19 weeks and -19.0 (+/- 9.3 SD)% at 39 weeks]. To assess the possibility of obtaining growth curves for volume parameters, 20 normal fetuses [based on crown-rump length (CRL), biparietal diameter (BPD), head circumference (HC) and abdominal circumference (AC) growth curves, birth weight (WT), crown-heel length (CHL) and postnatal examination] with known dates of conception were studied. Head volume (HV), thoracic volume (TV), abdominal volume (AV), limb volume (LV) and TOTV were measured at 2- to 3-week intervals during the third trimester. Mathematical modeling indicated that the linear-cubic (LC) model was optimal for HV, TV, AV, and TOTV (R2: 97.3%, 97.2%, 98.1% and 99.4%) while the linear-quadratic (L-Q) model was optimal for LV (R2: 98.4%). Significant individual variation was suggested by the high coefficients of variation (CV) obtained for the optimal model coefficients. Inspection of individual growth curves revealed the presence of three types, "linear," "concave," and "convex" with the "concave" type predominating. The variability was less than that seen in the groups as a whole for most subgroups but was still greater than that observed with linear parameters. These results indicated that although volume growth curves can be obtained, their individual variability is significant and thus more fetuses must be studied before standard curves can be defined.