Ultrasonographic measurement of thymus size in IUGR fetuses: a marker of the fetal immunoendocrine response to malnutrition.

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) <5(th) 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 <5(th) 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-to-expected mean for gestation thymus perimeter ratio and the enrollment-to-delivery interval (r = 0.44, P < 0.001).

Conclusions: 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.