OC8.03: Orbital measurements at 11–14 weeks of gestation

Materials and methods: The maximum transverse diameter of the thymus was measured in 376 normal fetuses from 19 to 38 weeks of gestation. Mean diameter and standard deviation at each gestational week was calculated. We assessed the relationship of the thymic diameter with gestational age (GA), biparietal diameter (BPD), femur length (FL), and abdominal circumference (AC). Linear regression of transverse diameter on GA, BPD, FL, and AC was calculated. Results: Measurements of the transverse diameter were possible in 352 of 376 fetuses (93.6%). The transverse diameter of the fetal thymus increased according to the fetal GA and size. The regression equations for transverse diameter of the thymus as a function of GA, BPD, FL, and AC was: thymic diameter (cm) = −1.58 + 0.15 × GA (weeks), −1.038 + 0.51 × BPD (cm), −0.63 + 0.63 × FL (cm), and −0.74 + 0.14 × AC (cm). Conclusion: We could get the reference value of the transverse diameter of the fetal thymus at each gestational week. The ultrasound measurement of transverse diameter is useful method to assess the development of the fetal thymus.