An operation to begin separating 3-year-old twin girls joined at the head was halted because one girl's brain was swollen, a hospital official said Thursday.

When medications to reduce the swelling were ineffective Wednesday, doctors closed an opening to Tatiana and Anastasia Dogaru's brains without beginning to separate them, said Nathan Levitan, chief medical officer at University Hospitals Case Medical Center and the leader of the surgery team.

The twins, born in Italy to Romanian parents, were awake and alert a day after the operation, he said.

The medical team was to conduct studies including an MRI to determine the cause of the swelling, then decide whether they could attempt the procedure again, Levitan said.

After the plastic surgery team at University Hospitals' Rainbow Babies & Children's Hospital removed a section of bone, doctors discovered the brain tissue of Anastasia, the larger and stronger twin, was swollen. Doctors also noted that her blood pressure was lower than usual.

"They felt that it would not be safe to cut into the brain tissue or the surrounding blood vessels in any way, without first understanding the cause of the swelling as well as the slightly low blood pressure," Levitan said.

The surgery, one of four procedures planned over several months, lasted most of Wednesday and didn't end until early evening.

The top of Tatiana's head is attached to the back of Anastasia's, and they have never been able to look directly at each other.

The girls have already beaten the odds by living this long. Most twins joined at the head die at birth and just 10 percent survive to age 10, according to the hospital.

The parents, the Rev. Alin Dogaru, a Byzantine Catholic priest, and Claudia Dogaru, both 31, have said they view the separation surgeries as the girls' best hope. They arrived in Cleveland in April after 2 1/2 years in Dallas.