Intrauterine growth of normal thyroid.

In this study we describe fetal thyroid growth during gestation and establish normal reference values using a simple linear ultrasound measurement of the thyroid. A total of 1180 normal singleton pregnancies, with no known risk factors for thyroid disorders, were enrolled from 12 to 39 weeks of gestation. The thyroid antero-posterior diameter was measured on a transverse axial plane through the fetal neck. The best fit regression was a power equation: thyroid diameter = 0.2412 weeks(1.0278) (r(2) = 0.55). The percentiles smoothed curves were calculated for each week. In conclusion, the results of the present study support previous findings that the fetal thyroid grows between 12 and 39 weeks of gestation with a steepest increase during the second trimester, that is when the fetal thyroid becomes functionally active. The normal ranges of this simple index of thyroid growth can be useful both as screening and for the clinical evaluation of pregnant patients with thyroid disorders.