This study examined rates of concomitant structural and chromosomal abnormalities in 14 fetuses with a diagnosis of Dandy-Walker malformation or Dandy-Walker variant before 21 weeks' gestational age, compared to 14 fetuses with a diagnosis of Dandy-Walker malformation or variant between 21 weeks' gestation and delivery. A total of 24 fetuses had Dandy-Walker malformation and four had Dandy-Walker variant. Eight of the fetuses with the malformation had ventriculomegaly: one of the fetuses with early diagnosis and seven with later diagnosis (p = 0.027). None of the fetuses with Dandy-Walker variant had ventriculomegaly. The overall prevalence of concomitant structural abnormalities was 13/28; 8/14 for fetuses with early prenatal diagnosis and 5/14 for fetuses with late prenatal diagnosis of the malformation or the variant. Chromosomal abnormality rates were significantly higher among fetuses with early prenatal diagnosis (7/14) than among those with later prenatal diagnosis (1/14; p = 0.032). Abnormal karyotypes were more prevalent among fetuses without ventriculomegaly (7/20), compared to fetuses with ventriculomegaly (1/8). We conclude that fetuses with an antenatal diagnosis of Dandy-Walker malformation or Dandy-Walker variant before 21 weeks' gestational age have worse prognosis than fetuses with a later prenatal diagnosis of the same defect.