Scalp neoplasm associated with cranium bifidum in a 24-week human fetus.

Prenatal ultrasonography at 24 weeks disclosed a fetus with a large mass protruding from the occiput. The mass was an invasive, undifferentiated mesenchymal neoplasm of the scalp associated with a defect in the occipital bone (cranium bifidum). The tumor may have derived from neural crest at the site of rhombencephalic closure. The tumor itself would seem to be a sporadic event, but the cranium bifidum may imply a recurrence risk of 2-3% for future pregnancies.