Differences in fat and lean mass proportions in normal and growth-restricted fetuses

Objective The purpose of this study was to assess fetal subcutaneous fat and lean mass areas as predictors of fetal growth restriction. Study design Seventeen severe fetal growth-restricted (abdominal circumference, <2 SD) fetuses and 20 control fetuses underwent ultrasound measurements of subcutaneous fat, lean mass, and standard biometry. Thigh subcutaneous fat and lean mass were measured on enlarged ultrasound axial images (subcutaneous fat area = total cross sectional area – lean mass area [bone + muscle areas]). Subcutaneous fat and lean mass areas were expressed as a percentage of the total cross-sectional area and were normalized to femur length and head circumference. Measurements were modeled as a function of fetal growth-restriction status and week of gestation with multiple linear regression. Results Fetal growth-restriction fetuses showed reductions in fat and lean mass (in standard biometry) and showed a disproportionate reduction in fat mass compared with lean mass. These were all associated significantly with fetal growth restriction. Conclusion Fetal growth-restricted fetuses have reduced subcutaneous fat and lean mass compared with control fetuses; a further reduction occurs in subcutaneous fat concentration compared with the reduction in lean mass when fat is normalized for body size, with either head circumference or femur length. Fat-to-bone proportions may be useful in distinguishing the small for gestational age fetus who is truly fetal growth restriction from the constitutionally small fetus.