Normal fetal thyroid volume.

A prospective study of 289 normal fetal thyroids was undertaken between 20 and 36 weeks of gestation in 289 mothers without any history of thyroid disease or maternal disease that could cause intrauterine growth restriction. The fetal thyroid volume was calculated by the ellipsoid equation, with the assumption that each lobe was an ellipse. A nomogram relating fetal thyroid volume to gestational age was established. The mean fetal thyroid volume was found to be 0.23 +/- 0.18 cm3 (median 0.17 cm3; range 0.022-0.85 cm3). The ratio of fetal thyroid volume to estimated fetal weight (V/W ratio) (at 0.163 +/- 0.079 cm3/kg) was constant throughout gestation. The value was smaller than those of the child and the adult reported in other studies, indicating that fetal thyroid enlargement is likely to be underestimated if based on postnatal criteria. It was found that the mean fetal thyroid volume was significantly correlated with estimated fetal weight and gestational age, in which the former dominated the effect. The fetal thyroid increased in size much faster after 32 weeks, suggesting that evaluation of possible fetal goiter should take into account gestational age and the nomogram developed in this study.