LONDON (AP) -- Faith was breathing for Hope. So when the newborn conjoined Williams twins were separated, it turned out that Hope couldn't live without her sister.
Week-old Hope Williams died Tuesday night at London's Great Ormond Street Hospital after being separated from her sister, her doctor, Agostino Pierro, said Wednesday.
"Hope's lungs were too small to support ... breathing and the circulation," said Pierro, who headed the surgical team that separated the twins. "The lungs of Faith were somehow supporting Hope."
Faith was in stable condition Wednesday in the hospital's intensive care unit. Her chances of survival are about 50 percent, Pierro said, and she will have to undergo further surgery to close the incision in her stomach.
The twins were born Nov. 26 by Caesarean section, joined from their chests to the lower part of their stomachs. They had separate hearts, but shared a liver and intestines.
Doctors had hoped to give the girls more time to become stronger before trying to separate them, but Pierro said the infants began to deteriorate Monday.
"This was an emergency operation, because there was a blockage in their joint intestine, which could only be resolved through surgery," he said.
A team of more than 20 doctors, nurses and other health care professionals worked to separate Faith and Hope in an operation that began Tuesday morning and finished 11 hours later.
"The operation done on Hope and Faith was one of the most complex and challenging we have ever faced," Pierro said.
Hope's parents, 18-year-old Laura Williams and her husband, Aled, from Shropshire in west England, were with their daughter when she died.
"They are clearly devastated by the loss of their daughter," Pierro said.
Twins joined at the abdomen are easier to separate than those joined elsewhere, such as the head. Last month, doctors in Cleveland, Ohio decided it would have been too risky to separate 4-year-old Italian twins born connected at the head. The girls, Anastasia and Tatiana Dogaru, will remain together, doctors said.
Dr. Charles Stolar, pediatric surgeon in chief at Morgan Stanley Children's Hospital of New York-Presbyterian Hospital, said if all goes well, Faith's recovery should take weeks rather than months.
"They've had to divide the liver, they've had to repair bile ducts, they've had to repair intestines," said Stolar, who has performed similar operations. "So all that has to heal ... I would be optimistic."