(CNN)Here's a look at conjoined twins.
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. About half of the twins 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 - 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 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 begins a successful 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.
Faith and Hope Williams
Born November 26, 2008 in Great Britain. They are attached from their chests to the lower part of their stomachs.
December 2, 2008 - Undergo an 11-hour surgery to separate them. Hope dies just hours later.
December 25, 2008 - Faith dies from complications of the separation and subsequent surgeries.
Preslee Faith and Kylee Hope Wells
Born October 25, 2008 in Oklahoma City, Oklahoma 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, Oklahoma. Both survive though are listed in critical condition after the surgery.
February 19, 2011 - Preslee Faith Wells dies at The Children's Center in Bethany, Oklahoma..
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, San Jose, California.
Hassan and Hussein Benhaffaf
Born December 2, 2009 in London to Angie and Azzedine Benhaffaf from east Cork, Ireland. They are attached at the chest but share no major organs.
April 8, 2010 - Undergo a 14-hour separation surgery at Great Ormond Street Hospital. Both survive.
Maria and Teresa Tapia
Born April 8, 2010 in the Dominican Republic to Lisandra Sanatis and Marino Tapia. They are joined at the lower chest and abdomen and share a liver, pancreas glands, and part of their small intestine.
November 8, 2011 - Are successfully separated following a 20-hour procecdure.
Joshua and Jacob Spates
Born January 24, 2011 in Memphis, Tennessee to Adrienne Spates. They are joined back to back at the pelvis and lower spine, each with separate hearts, heads and limbs.
August 29, 2011 - Are successfully separated after a 13-hour surgery.
October 2013 - Jacob Spates passes away. The cause of his death is not released.
Rital and Ritag Gaboura
Born September 22, 2010 in Khartoum, Sudan to Abdelmajeed and Enas Gaboura. They are joined at the head.
August 15, 2011 - Are successfully separated after a four-stage operation. Two operations took place in May, one in July and the final operation in August.
Allison June and Amelia Lee Tucker
Born March 1, 2012 to Shellie and Greg Tucker. They are attached at the lower chest and abdomen and share their chest wall, diaphragm, pericardium and liver.
November 7, 2012 - Are successfully separated after a seven-hour surgery.
A'zhari and A'zhiah Lawrence
Born October 10, 2012 in Virginia to Nachell Jones and Carlos Lawrence. They are joined from the chest to the abdomen and have a conjoined liver.
October 25, 2012 - An emergency surgery is performed to separate their liver.
April 22, 2013 - Are successfully separated following 14 hours of sugery.
October 14, 2013 - A'zhari passes away.
Emmett and Owen Ezell
Born July 15, 2013 in Dallas, Texas, to Jenni and Dave Ezell. They are joined at the liver and the intestine.
August 24, 2013 - Are successfully separated.
Asa and Eli Hamby
Born December 4, 2014 in Atlanta, Georgia, to Robin and Michael Hamby. They share a torso, heart, arms and legs.
December 5, 2014 - Asa and Eli pass away.
Carter and Conner Mirabal
Born December 12, 2014 in Jacksonville, Florida, to Michelle Brantley and Bryan Mirabal. They are joined at the sternum and abdomen and share a liver and part of their small intestines.
Scarlet and Ximena Torres
Born May 16, 2015 in Corpus Christi, Texas to Silvia Hernandez and Raul Torres. Hernandez gave birth to rare identical triplets with Scarlet and Ximena conjoined at the pelvis.