Fri Jan 30, 2:01 PM ET

SAN ANTONIO, Texas (Reuters) - Six-month-old twin girls, joined from their chests to their abdomens, have died after doctors concluded there was no possibility that they would survive separation, a hospital spokesman said on Friday.

Brynleigh Belle and Victoria Grace Smith died on Thursday after their parents requested they be taken off the ventilators that had helped keep them alive since they were born on July 25, 2003, said a spokesman at Wilford Hall Medical Center at Lackland Air Force Base.

Joined from just below their collarbones to their bellies, each of the girls had two arms and two legs but they shared a heart, liver, diaphragms and parts of their intestines.

Doctors said the complexity of their shared anatomy made it impossible to separate the girls and have them survive.

Physicians also said that if they were not separated, they would have to battle many medical complications and would likely never come off of ventilators.

The girls' health had declined recently as they fought infections and damage to their lungs and livers.

Their parents, Dawn and Matt Smith, were at their side when they died.

"We had to look at the whole picture, at the whole package," their mother, Dawn Smith, told the San Antonio Express-News.

"They are incredibly sick. They can't get off the ventilators. I think they are just telling me, 'Mommy, mommy, I'm tired. I want to rest. I want to go home."'

The parents attended memorial ceremonies for the girls on Friday