Comparison of 3D fetal thigh fractional limb volume between two Fetal Growth Restriction (FGR) cohorts

3D Fractional limb volume (FLV) measurements of the fetal thigh correlate well with neonatal percent body fat in normal weight neonates. As 80% of fetal fat accumulates in the third trimester, FLV measurements of subcutaneous fat may identify whether small fetuses are normal or pathologically small. The objective of this study was to compare FLV in a control group of growth-restricted (FGR) fetuses (EFW <10th percentile) to a FGR group meeting a strict definition of FGR per 2016 ISUOG criteria (either an EFW or AC <10th percentile with abnormal umbilical artery Dopplers (UA PI >95thile or CPR <5thile for GA), or, an EFW or AC <3rd percentile alone). In this prospective cohort study, 45 fetuses identified as FGR (EFW <10th%ile at enrollment) were divided into two groups: those meeting ISUOG criteria (FGR-I; n=25) selecting a more pathologic cohort, and FGR controls (FGR-C; n=20) not meeting strict criteria. The final ultrasound exam prior to delivery (∼35 weeks) was used to obtain biometry, EFW, and 3D FLV acquisition. 3D FLV was completed by estimating the total thigh volume (TTV) along the femur using a 5-slice technique, as well as subcutaneous fat (SQ) and lean mass (LM; bone + muscle) volume assessments. Compared to the FGR-C group, the FGR-I group delivered at a similar GA, but with a significantly lower birth weight. Compared to the FGR-C group, the FGR-I group showed significant reductions in TTV, SQ, and LM (Table 1). The significant difference in birth weights between the FGR-C and FGR-I groups who delivered at a similar gestational age validates our two comparison groups. The significant difference in TTV, SQ, and LM found between the two groups suggests that the FGR group meeting ISUOG criteria (FGR-I) may represent a more pathologic phenotype of FGR. Use of the FLV when evaluating FGR pregnancies may improve the ability to distinguish between normal and pathologic FGR. (Funded by the Perelman IUGR Study)