Second trimester growth velocities: assessment of fetal growth potential in SGA singletons

To evaluate the validity of second trimester growth velocities as measures of fetal growth potential in Small-for-Gestational-Age (SGA) singletons.

Second trimester growth velocities for biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur diaphysis length (FDL) were determined by linear regression analysis or direct measurement in 53 SGA singletons with normal growth outcomes (SGA N Group) and 73 with growth restriction (SGA GR) based on a composite fetal growth pathology score (FGPS1). The latter were subdivided into six groups based on their growth restriction pattern (Patterns group). Similar data were available for 118 singletons with normal neonatal growth outcomes (NNGO group). Coefficients of determination (R²) and growth velocities for each anatomical parameter were compared between Patterns subgroups and the SGA N, SGA GR and NNGO groups.

Median R² values in the six Patterns subgroups ranged from 98.2% (Pattern 2, FDL) to 99.9% (Pattern 5, AC). Within each anatomical parameter set, no significant differences were found (Kruskal-Wallis). Patterns subgroup data were pooled to form the SGA GR group for each anatomical parameter. Mean values for the three main groups ranged from 98.4% (SGA N, FDL) to 99.6% (SGA N, HC). No significant differences between groups (ANOVA) were found for any anatomical parameter (ANOVA). Only 1.7-3.8% had R² values <95th%. No significant differences in median second trimester growth velocities among different Patterns subgroups were found for any anatomical parameter. In the SGA N and SGA GR groups, mean BPD and HC values did not differ but were significantly smaller than the NNGO group values. No differences in mean FDL values were seen. With AC, all three means were significantly different, having the following order: NNGO > SGA N > SGA GR. Of all 504 second trimester growth rates, 92.5% were within their respective 95% reference ranges.

Growth in the second trimester is linear in fetuses at risk for growth restriction. Except for FDL, growth velocities were lower than those for fetuses with NNGO. Only AC had mean velocities that differed between the SGA N and the SGA GR groups. Since most velocities (92.5%) were within normal reference ranges, they are reasonable measures of growth potential in fetuses at risk for growth restriction.