Re: Why does a fertilized egg split after conception?

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.