Ultrasonography of the fetal thyroid - Nomograms based on biparietal diameter and gestational age

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. Nomogra ms of fetal thyroid size were created by using the 5th, 10th, 50th, 90th, a nd 95th percentiles based on biparietal diameter and gestational age. A sec ond-order polynomial fit for biparietal diameter and a linear fit for gesta tional age best described thyroid circumference measurements. Variations in thyroid circumference measurements increased with both larger biparietal d iameter and advancing gestational age. There was no intraobserver or intero bserver variability in thyroid circumference measurements (P > .20). Conclu sions. Both biparietal diameter and gestational age serve as good predictor s of fetal thyroid circumference. When the biparietal diameter is difficult to measure, gestational age can be used to assess thyroid size.