I searched the Internet and there doesn't SEEM to be an explanation of why THIS disorder is usually fatal. So many other disorders aren't.
I’m looking for an answer. Here’s the first input I found:
“In addition to facial duplication, anencephaly, neural tube defect and cardiac malformations represent the more common congenital abnormalities associated with diprosopus. The pathogenesis of diprosopus is not well understood.”
[http://www.ncbi.nlm.nih.gov/pubmed/10048094]
From that it sounds as if the condition necessarily entails other deformities. Why, as the quote explains, is not clear.
Second input (which yes, is Wiki, but on non-political topics Wiki sometimes makes sense):
‘Although classically considered conjoined twinning (which it resembles), this anomaly is not normally due to the fusion or incomplete separation of two embryos. It is the result of abnormal activity by the protein SHH (sonic hedgehog).[7][8][9] (The name of this protein was inspired by the Sonic the Hedgehog videogame character and is part of an idiosyncratic naming tradition in molecular biology research.)[10][11]”
“Diprosopus often occurs in combination with other congenital disorders, particularly anencephaly, neural tube defect and cardiac malformations.[14] When present, the brain may show abnormalities ranging from partial to complete duplication of brain structures, and/or underdevelopment of brain tissues.[15][16]”
“Most human infants with diprosopus are stillborn. Known instances of humans with diprosopus surviving for longer than minutes to hours past birth are very rare; only a few are recorded. In 2002 and 2003, two living male infants with partial diprosopus were described in the medical literature in separate case reports.[20][21] One infant was born with duplication of the nose and the cerebral frontal lobes, two widely spaced eyes, a small, underdeveloped central eye socket, and a large, asymmetric mouth. The other infant was born with duplication of the upper and lower jaw, two tongues ending in the same base, cleft palate, a slightly divided tip of the nose, and two widely spaced eyes, as well as absence of the corpus callosum, duplication of the pituitary gland and stalk, and abnormalities in the midbrain. Because they were born with a milder, partial form of diprosopus, both infants were considered candidates for surgical correction of their abnormal facial features.”
[http://en.wikipedia.org/wiki/Diprosopus]
So what I’m gathering is that an abnormal development (caused by the SHH protein) is what causes this condition. I.e.: rather than being conjoined twins, Diprosopus is by nature a type of abnormal twinning. It’s too rare to be well quantified, but if you go to the Wiki article you’ll see two cases where survival (in human babies) has extended for years so far. Perhaps that will be the case with Hope & Faith. At any rate, they received the gift of birth/life, as opposed to a horrible death in the womb. We can all thank God for that.