Conjoined Twins

What causes conjoined twins?

There are two theories about the development of conjoined twins. The first is that a single fertilized egg does not fully split during the process of forming identical twins. The second theory is that a fusion of two fertilized eggs occurs earlier in development. Although conjoined twinning has not been linked to any environmental or genetic cause, they occur so rarely it has not been possible to draw firm conclusions. 

How common are conjoined twins?

The frequency of the occurrence of conjoined twins is not well recorded, but is estimated to be between 1 in 30,000 to 1 in 200,000 worldwide. Because they occur so rarely, it is difficult to determine an exact frequency. The majority of conjoined twins die in utero, and about half who are born alive do not survive for more than 24 hours.

How often is separation surgery performed?

Separation surgery is performed in the United States about five times a year. Most recently, in October 2016, physicians at the Children’s Hospital at Montefiore Medical Center in New York separated 13-month-old conjoined twins who were joined at the head.

Has a separation surgery ever been performed at Lucile Packard Children’s Hospital Stanford?

Three sets of conjoined twins have been separated at Packard Children’s. On December 7, 2016 doctors successfully completed the separation of 2-year-old twins Eva and Erika Sandoval. Prior to that, conjoined twins were separated at Packard Children’s in 2007 and 2011. Gary Hartman, MD, has led seven total separation surgeries of conjoined twins, including these three cases.