Objective To test the hypothesis that intrauterine growth restriction (IUGR) is associated with decreased thymus size in the human fetus. Methods The thymus perimeter was measured in 60 consecutive IUGR fetuses at prenatal ultrasound examination. IUGR was defined as an abdominal circumference (AC) <5th centile. Sixty controls were identified by selection of the next consecutive appropriately grown fetus of similar gestational age (±1 week). To exclude fetal size effects, ratios between thymus perimeter and fetal biometry measurements including biparietal diameter (BPD), AC and femur length (FL), as well as estimated fetal weight (EFW) were compared between IUGR fetuses and controls. Results The proportion of fetuses with thymus perimeter <5th centile for gestation was significantly higher in IUGR fetuses than in controls (58/60 vs. 7/60, P < 0.0001). The mean thymus perimeter/BPD ratio (0.87 ± 0.20 vs. 1.13 ± 0.13, P < 0.0001), thymus perimeter/AC ratio (0.28 ± 0.06 vs. 0.35 ± 0.03, P < 0.0001), thymus perimeter/FL ratio (1.18 ± 0.26 vs. 1.51 ± 0.19, P < 0.001) and thymus perimeter/EFW ratio (0.05 ± 0.01 vs. 0.06 ± 0.01, P = 0.02) were significantly lower in IUGR fetuses than in controls. There was a significant positive correlation between the observed-toexpected mean for gestation thymus perimeter ratio and the enrolment-to-delivery interval (r = 0.44, P < 0.001). Conclusion IUGR is associated with a disproportionately small thymus. This supports the hypothesis that thymic involution may be part of the fetal neuroendocrine response to intrauterine starvation.
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