We present transvaginal ultrasonographic findings of a fetus with Dandy-Wal
ker malformation and associated massive obstructive hydrocephalus at 13 wee
ks' gestation. First-trimester ultrasonographic diagnosis of Dandy Walker m
alformation is uncommon with only two such occurrences having been reported
previously. These cases and recent reports of single gene transmission of
this condition in some families emphasize the importance of first-trimester
transvaginal ultrasound assessment especially in women with previously aff
ected fetuses.