Conjoined Twins: What went wrong?

My cousins are identical twins. I actually wasn't able to tell them apart until they were 17.

Identical twins share more than their matchy-matchy outfits. Identical twins, or monozigotic twins, start as one fertilized egg, which later splits into two. This causes their DNA to be the same. Two exact copies.

So where does it go wrong for conjoined twins?

In the case of conjoined twins, a woman only produces a single egg, which does not fully separate after fertilization. The developing embryo starts to split into identical twins during the first few weeks after conception, but stops before the process is complete. The partially separated egg develops into a conjoined fetus.

Abigail and Brittany Hensel, born March 7, 1990 in Carver County, Minnesota, United States of America, dicephalic conjoined twins, two heads, two arms, two legs, cannot be separated.

About 40 to 60 percent of conjoined twins are still born, they die during birth. And, for some reason, female siblings seem to have a better shot at survival than their male counterparts. Although more male twins conjoin in the womb than female twins, females are three times as likely as males to be born alive. Approximately 70 percent of all conjoined twins are girls.

Surgical separation is tough one. Doctors need to see which organs the conjoined twins share. Also, the difficulty level rises depending on where the twin is joined. For example, twins joined at the sacrum at the base of the spine have a 68 percent chance of successful separation, whereas, in cases of twins with conjoined hearts at the ventricular (pumping chamber) level, there are no known survivors.

Although success rates have improved over the years, surgical separation is still rare. Since 1950, at least one twin has survived separation about 75 percent of the time.

Even after survival of surgical separation, twins will have to undergo intensive rehabilitation because of the malformation and position of their spines. The muscles in their backs are constantly being flexed and they often have a difficult time bending their backs forward and backwards and sitting up straight.


Source: http://www.umm.edu/conjoined_twins/facts.htm