Fetal body composition reference charts and sexual dimorphism using magnetic resonance imaging

The American Academy of Pediatrics advises that the nutrition of preterm infants should target a body composition similar to that of a fetus in utero. Still, reference charts for intrauterine body composition are missing. Moreover, data on sexual differences in intrauterine body composition during pregnancy are limited.

The objective of this study was to create reference charts for intrauterine body composition from 30 to 36+6 weeks postconception and to evaluate the differences between sexes.

In this single-center retrospective study, data from 197 normal developing fetuses in late gestation was acquired at 3T magnetic resonance imaging (MRI) scans, including True Fast Imaging with Steady State Free Precession and T₁-weighted 2-point Dixon sequences covering the entire fetus. Deep convolutional neural networks were utilized to automatically segment the fetal body and subcutaneous adipose tissue. The fetus's body mass (BM), fat signal fraction (FSF), fat mass (FM), FM percentage (FM%), fat-free mass (FFM), and FFM percentage (FFM%) were calculated. Using the Generalized Additive Models for Location, Scale, and Shape (GAMLSS) method, reference charts were created, and sexual dimorphism was examined using analysis of covariance (ANCOVA). A P value <0.05 was deemed significant.

Throughout late gestation, BM, FSF, FM, FM%, and FFM increased, while the FFM% decreased. Reference charts for gestational age and sex-specific percentiles are provided. Males exhibited significantly higher BM (7.2%; 95% confidence interval [95% CI]: 1.9, 12.4), FFM (8.8%; 95% CI: 5.8, 11.9), and FFM% (1.7%; 95% CI: 1, 2.4) and lower FSF (-3.6%; 95% CI: -5.6, -1.8) and FM% (-1.7%; 95% CI: -2.4, -1), (P < 0.001) compared with females, with no significant difference in FM between sexes (P = 0.876).

MRI-derived intrauterine body composition growth charts are valuable for tracking growth in preterm infants. This study demonstrated that sexual differences in body composition are already present in the intrauterine phase.