Seen in 1.0/10,000 births, holoprosencephaly is a serious brain abnormality usually causing mental retardation and/or death. It arises around 4 weeks after conception, when the developing forebrain fails to undergo normal division into 2 lobes. This area of the brain—the site of intellectual function—is closely linked with midfacial development. The spectrum of facial findings seen with holoprosencephaly includes cyclops (single eye), closely spaced eyes with a single nostril, midline cleft lip and/or a single central front incisor tooth.
Our interview study of pregnancy factors in chromosomally normal babies with holoprosencephaly indicates: