Fetal thymus size in human pregnancies reveals inverse association with regulatory T cell frequencies in cord blood

OBJECTIVE: To determine fetal thymus growth and its relationship with fetal weight and cord blood T-regulatory cells in a prospective study. Assessment of fetal immune organs by ultrasound could provide a screening approach to identify fetuses at risk of impaired postnatal immunity.

STUDY DESIGN AND OUTCOME MEASURES: Thymus size was measured with four ultrasound techniques. The approaches with lowest coefficient of variation (thymus transverse diameter, 3 vessel edge) were used to longitudinally assess fetal and thymus growth in 137 cases at four time points between a gestational age (GA) of 13 and 37 weeks. Cord blood at birth was analyzed by flow-cytometry to evaluate the frequency of regulatory T (Treg) cells.

RESULTS AND CONCLUSION: Fetal thymus growth is significantly correlated with fetal weight (GA 23-25 weeks r=0.40, p<0.01; GA 28-30 weeks r=0.21, p=0.04, GA 35-37 weeks r=0.56, p<0.01). We observed an inverse correlation between fetal thymus size at GA 23-25 weeks and cord blood Treg cells (r=0.37, p=0.01). Thymus growth occurs in a linear fashion throughout pregnancy and can be reliably measured using ultrasound. Our findings of an inverse correlation between thymus growth and Treg cells in cord blood suggests a link between fetal growth, thymus development and immune-status at birth.