Objective: To investigate the value of prenatal ultrasound measurement of thymus size and to determine whether maternal administration of euthyrox affects fetal thymus development. Methods: A total of 311 healthy pregnant women(18-40 weeks of gestation)(healthy Group) were enrolled,and prenatal ultrasound examination were performed once in each pregnancy in the department of ultrasound of the hospital from January 2018 to January 2021.In addition, 31 pregnant women (14 cases at 28 weeks and 17 cases at 34 weeks) who took euthyrox(euthyrox Group) for hypothyroidism were selected. The morphology, echo and size of fetal thymus were observed and the anteroposterior diameter, transverse diameter, circumference and area of fetal thymus were measured by 2D-ultrasound;The correlation between ultrasonic parameters and gestational age in healthy group and was analyzed. The differences in thymus related parameters between the euthyrox Group and health Group were compared to determine if maternal administration of euthyrox may affect fetal thymus development. Results: The parameters of fetal thymus (the anteroposterior diameter, transverse diameter, circumference and area)in the healthy group were linearly correlated with gestational age. There was no significant difference in parameters of fetal thymus between the euthyrox group and the healthy group during the same period. Conclusions: 2D-ultrasound may serve as an effective approach to assess prenatal thymus development,and maternal administration of euthyrox had no effect on fetal thymus development.