Here's a look at what you need to know about conjoined twins.
General Information: Conjoined twins are physically connected to one another at some point on their bodies.
Conjoined twins occur once every 200,000 live births.
About 70% of conjoined twins are female.
Conjoined twins are identical - they are the same sex.
The form of conjoined twinning refers to the point where the twins are joined. The top five, starting with the most frequent: thoracopagus (heart, liver, intestine), omphalopagus (liver, biliary tree, intestine), pygopagus (spine, rectum, genitourinary tract), ischiopagus (pelvis, liver, intestine, genitourinary tract), and craniopagus (brain, meninges). (American Pediatric Surgical Association)
Scientists believe that conjoined twins develop from a single fertilized egg that fails to separate completely as it divides.
The overall survival rate of conjoined twins is between 5% and 25%, with about 75% of surgical separations resulting in at least one twin surviving.
An estimated 250 surgical separations have been performed worldwide, with most performed since 1950.
Approximately 200 pairs of conjoined twins are born alive each year, and about half die before their first birthday.
The term "Siamese twins" originated with Eng and Chang Bunker, a set of conjoined twins who were born in Siam (now Thailand) in 1811. They lived to age 63 and appeared in traveling exhibitions. Chang and Eng both married and fathered a total of 21 children between them.
Selected Recent Cases:
Lea and Tabea Block Born August 9, 2003 in Lemgo, Germany, to Peter and Nelly Block. They are joined at the head.
September 16, 2004 - at 12:15am, the girls are separated. Tabea dies shortly thereafter.
Jade and Erin Buckles Born February 26, 2004 - to Melissa and Kevin Buckles at National Naval Medical Center in Bethesda, Maryland. They share a liver.
June 19, 2004 - They are successfully separated at four and one-half months.
Tatiana and Anastasia Dogaru Born January 13, 2004 - in Rome, Italy to Romanian parents Claudia Alin and Claudia Dogaru. They are connected at the head.
June 7, 2007 - Scheduled surgery to separate the girls is halted after doctors notice brain swelling in one girl.
August 2007 - Doctors at University Hospitals Case Medical Centre Rainbow Babies & Children's Hospital in Cleveland, Ohio announce that they won't proceed with separation of the girls, because the surgery is too risky.
Abbigail and Isabelle Carlsen Born November 29, 2005 - in Fargo, North Dakota, to Amy and Jesse Carlsen. They are joined at the abdomen and chest.
May 12, 2006 - A team of 30 people, including 18 surgeons from various specialties, at the Mayo Clinic begin a 10-12 hour operation to separate the girls.
Regina and Renata Salinas Fierros Born August 2, 2005 - in Los Angeles, California, to Sonia Fierros and Federico Salinas. Born facing each other and joined from the lower chest to the pelvis, they are fused in several places, including the liver and genitals, they share a large intestine, and Regina is born with one kidney.
June 14, 2006 - The twins are separated in a day-long surgery at Children's Hospital Los Angeles.
Abygail and Madysen Fitterer Born August 8, 2006 - to Stacy and Suzy Fitterer, from Bismarck, North Dakota. They are born joined at the abdomen, sharing a liver.
January 3, 2007 - Separated in a surgery at the Mayo Clinic in Rochester, Minnesota.