A fetus with Down syndrome and intratubular germ cell neoplasia.

A 22-week-old fetus with trisomy 21 demonstrated intratubular germ cell neoplasia. This is the second report of in situ testicular neoplasia in a fetus with trisomy 21, suggesting that the mechanism responsible for the possible excess of testicular germ cell tumors that occur in Down syndrome is operative in early fetal life. Because no examples of neonatal testicular germ cell tumor in trisomy 21 have been reported, we suggest that this in situ neoplasm may disappear during gestation, such as the trisomy 21-associated transient myeloproliferative syndrome does shortly after birth. This disappearance may be due to a controlled regression phenomenon.