Doctors performed the first of a lengthy series of high-risk surgeries to separate 3-year-old twin girls joined at the head.
The initial surgery lasted most of Wednesday and did not end until early evening.
Tatiana and Anastasia Dogaru, ethnic Romanians born in Rome, were under close observation in a pediatric intensive care unit and no update on their conditions was expected until Thursday, with the first detailed medical briefing expected Friday, said Janice Guhl, spokeswoman for University Hospitals' Rainbow Babies & Children's Hospital.
Doctors hope to separate the twins through several surgeries over a period of up to six months. Without separation, they risk dying in early childhood.
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 initial surgery was aimed at making a scalp wedge where the skulls are joined, creating a 2-inch by 8-inch rectangular bone flap - or kind of trap door to the brain - which can be closed after surgery.
The procedure offered neurosurgeons their first real glimpse of the girls' brains. The medical team practiced the procedure on a model designed from images.
The hospital assigned a photographer and videographer to the operating room and said some images might be released if the parents approve. There were satellite television trucks parked outside and visitors seemed aware of the emotion-charged occasion.
The parents, the Rev. Alin Dogaru, a Byzantine Catholic priest, and Claudia Dogaru, both 31, have said they viewed the separation surgeries as the girls' best hope. They arrived in Cleveland on April 6 after 2 1/2 years in Dallas.
Twins born joined at the head - known as craniopagus twins - are rare, occurring in about one in 2.5 million births.
Last month, doctors succeeded in establishing independent blood flow in the twins by inserting small coils into veins in their brains. That was viewed as a prerequisite for separation surgery.
Dr. Alan Cohen, chief of pediatric neurosurgery, and Dr. Arun Gosain, chief of pediatric plastic surgery, were confident a team of up to 50 specialists was ready. Surgeons spent Saturday and Monday in practice sessions.