COMPLICATIONS IN IDENTICAL TWINNING It is possible to have more complications with identical twinning because of the unusual splitting that takes place in the egg that produces identical twins, but most of these complications are rare. Types of problems include Siamese or conjoined twins, in which the twins are connected in various parts of their bodies. This happens only in identicals. This condition is rare and happens in one out of 33,000 births, with most of conjoined twins (about 70 percent) being female. Also, Twin-to-Twin Transfusion Syndrome (TTTS), another cause of problems in twin pregnancies, can occur only in identicals. Original text: http://www.nomotc.org/library/twinning_facts.html