Conjoined Twins Fast Facts

(CNN)Here's some background information about conjoined twins.

Conjoined twins are physically connected to one another at some point on their bodies.
    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). (Mayo Clinic)
    Scientists believe that conjoined twins develop from a single fertilized egg that fails to separate completely as it divides.
    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.
    In 1955, neurosurgeon Dr. Harold Voris of Mercy Hospital in Chicago performed the first successful procedure separating conjoined twins.

    Selected 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 - 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.
    Tatiana and Anastasia Dogaru
    Born January 13, 2004, in Rome to Romanian parents Claudia and Alin 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 of the girls.
    August 2007 - Doctors at University Hospital's Rainbow Babies & Children's Hospital in Cleveland announce that they will not 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 in Rochester, Minnesota, perform a successful operation to separate the girls.
    Regina and Renata Salinas Fierros
    Born August 2, 2005, in Los Angeles 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, and they share a large intestine. Regina is born with one kidney.
    June 14, 2006 - The twins are separated during 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 and share a liver.
    January 3, 2007 - Separated in a surgery at the Mayo Clinic.
    Preslee Faith and Kylee Hope Wells
    Born October 25, 2008, in Oklahoma City to Stevie Stewart and Kylie Wells. They are attached at the chest and are believed to be the first Native American conjoined twins.
    January 19, 2009 - Separated at Children's Hospital at OU Medical Center in Oklahoma City. Both survive but are listed in critical condition after the surgery.
    February 19, 2011- Preslee Faith Wells dies at The Children's Center in Bethany, Oklahoma.
    Arthur and Heitor Rocha Brandao
    Born April 2009 in Bahia, Brazil, to Eliane and Delson Rocha. They are joined at the hip and share a bladder, intestines, liver and genitals. The twins only have three legs between them.
    February 24, 2015 - The five-year-old twins undergo a 15-hour separation surgery after months of preparation. Arthur dies three days later after he suffers cardiac arrest.
    Angelica and Angelina Sabuco
    Born August 2009 in the Philippines to Fidel and Ginady Sabuco. They are joined at the chest and abdomen.
    November 1, 2011 - Are successfully separated after a 10-hour surgery at Lucile Packard Children's Hospital in San Jose, California.
    Hassan and Hussein Benhaffaf
    Born December 2, 2009, in Lond