There is an increased interest nowadays on ultrasound analysis of the fetal thymus. Abnormal fetal thymic growth have been associated with DiGeorge syndrome, conotruncal cardiac malformations, chromosomal abnormalities and adverse outcome in different perinatal conditions as intrauterine growth restriction, preterm birth and others. Different methodologies that measure the fetal thymus by ultrasound have been published, however there is not a consensus of which one is the most useful. Our aim is to describe these methodologies and discuss their clinical applications.