CLEVELAND - 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.

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

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

Doctors at University Hospitals' Rainbow Babies & Children's Hospital discovered the swollen brain tissue of Anastasia, the larger and stronger twin, after removing a section of bone. 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 medical team was to conduct tests including an MRI to determine the cause of the swelling, then decide whether they could attempt the procedure again, Levitan said.

"Even though no actual cutting into the brain tissue was done yesterday, some very important information was gained and I think our physicians were gratified to know that information," Levitan said.

"We remain hopeful that we can help the twins," he 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.

In a statement read by a hospital spokeswoman, the girls' parents said, "We are pleased to see the girls are in good shape. We know the doctors are being cautious to keep the girls safe. We want to thank you for your continued prayers."