Here is a nice example of the fetal thymus gland at 33 weeks of gestation. This gland sits anterior and superior to the heart between the sternum and three vessel view. The thymus can be hypoplastic or absent with VCF (22q deletion syndrome associated with conotruncal abnormalities of the heart). If you see an anomaly of the outlfow tracts, try to assess the size of the thymus. The following articles may be helpful, many more were found in a literature search so this is just a few resources:

"Diameter of the normal fetal thymus on ultrasound" Ultrasound Obstet Gynecol 2007;29:634-638 - this article has a nice line drawing similar to the image I am attaching...very helpful for those of us who are not too familiar with imaging the thymus

"Assessment of the thymus at echocardiography in fetuses at risk for 22q11.2 deletion"  Prenat Diagn 2003;23:9-15

"Assessment of the fetal thymus by two- and three-dimensional ultrasound during normal human gestation and in fetuses with congenital heart defects" Ultrasound Obstet Gynecol 2011;37:404-409

"The development of the fetal thymus: an in utero sonographic evaluation" Prenat Diagn 2002;22:114-117

"The thymic-thoracic ratio in fetal heart defects: a simple way to identify fetuses at high risk for microdeletion 22q11" Ultrasound Obstet Gynecol 2011;37:397-403 -This article has a nice line drawing to show the relative size and location of the thymus