|MadSci Network: Development|
Dear Louise, Unfortunately it is not known what causes a fertilized egg to split at the blastula stage and form monozygotic twins. Monozygotic (identical) twins are very rare in mammals, so it is a difficult area to research, although it is a very common phenomena in armadillos. Scientists have been able to induce monozygotic twinning in fish with oxygen deprivation, but it is unknown if this also is true for humans. Another interesting fact is that most conjoined twins are not a result of incomplete splitting of a fertilized egg. The egg does split, but the reason that the twins are joined is that they fuse back together after complete splitting. Thank you for your question, Sarah Martin Mason, Mad Scientist Reference Chitkara, Usha and Richard Berkowitz. Gabbe: Obstetrics- Normal and Problematic Pregnancy. Third Edition. 1996. Churchill and Livingstone.
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