Objective: To present the reference range of the fetal thymus gland according to gestational age groups. Methods: In this prospective study, fetal thymus size was assessed in singleton, uncomplicated pregnancies between 19 and 38 weeks of gestation in our outpatient clinic between 2019 and 2020. Based on their monthly pregnancy follow-ups, fetal thymus measurement was divided into 5 gestational age groups (Group 1: 19–22 weeks, Group 2: 23–26 weeks, Group 3: 27–30 weeks, Group 4: 31–34 weeks, and Group 5: 35–38 weeks). Results: Fetal thymus measurements of 210 patients were performed over one year, and as a result, 184 pregnant patients were included for assessment. Fetal thymus could be visualized at a rate of 93.5%. The 5th percentile of thymus transverse diameter, antero-posterior diameter, perimeter, thymus anterior-posterior diameter to thoracic diameter, and thymus perimeter to thoracic circumference were 11.03 mm, 5.60 mm, 32.52 mm, 0.33, and 0.32 in Group 1; 13.53 mm, 7.66 mm, 43.67 mm, 0.34, and 0.32 in Group 2; 20.43 mm, 11.22 mm, 47.72 mm, 0.33, and 0.32 in Group 3; 27 mm, 12.98 mm, 55.88 mm, 0.32, and 0.30 in Group 4; 28 mm, 13.59 mm, 63.4 mm, 0.32, and 0.30 in Group 5; respectively. Spearman’s rho correlation coefficients for the thymic measurements were 0.879, 0.869, 0.846, 0.236, and 0.267 respectively, and all p-values were less than 0.001. As a result of linear regression analysis between thymus measurements and BPD; the equations for the optimal models are as follows: thymus transverse diameter= -3.49+0.4×BPD (mm) (r=0.826, R2 =0.682, p<0.001), thymus anterior-posterior diameter= -2.48+0.22×BPD (mm) (r=0.808, R2 =0.653, p<0.001), thymus perimeter= -14.37+1.21×BPD (mm) (r=0.814, R2 =0.663, p<0.001), thymus anterior-posterior diameter /thoracic diameter= 0.38+7.76E-4×BPD (r=0.213, R2 =0.045, p=0.004) and thymus perimeter/thoracic circumference= 0.35+1.02E-3×BPD (r=0.263, R2 =0.069, p<0.001). Thymus transverse diameter, anterior-posterior diameter, and perimeter increased linearly with increasing biparietal diameter (BPD). Conclusion: We established the reference ranges of fetal thymus size. Thymus transverse diameter, antero-posterior diameter, and thymus perimeter have a strong relationship with gestational age and are easy and reproducible. Therefore, the knowledge of reference ranges of fetal thymus will enable the evaluation of thymic aplasia/hypoplasia