Objective: To describe gestational age-dependent and -independent nomograms for fetal thyroid size.
Methods: Two hundred fetuses were evaluated between 16 and 37 weeks' gestation in this cross-sectional study.
Results: Nomograms of fetal thyroid size were created by using the 5th, 10th, 50th, 90th, and 95th percentiles based on biparietal diameter and gestational age. A second-order polynomial fit for biparietal diameter and a linear fit for gestational age best described thyroid circumference measurements. Variations in thyroid circumference measurements increased with both larger biparietal diameter and advancing gestational age. There was no intraobserver or interobserver variability in thyroid circumference measurements (P > .20).
Conclusions: Both biparietal diameter and gestational age serve as good predictors of fetal thyroid circumference. When the biparietal diameter is difficult to measure, gestational age can be used to assess thyroid size.