Dallas ? Two-year-old twins from Egypt joined at the top of their heads will undergo separation surgery today at Children’s Medical Center of Dallas.

The surgery, starting in the morning and expected to last 18 to 24 hours, comes more than a year after the boys arrived in Dallas for an evaluation to determine the risks for separation.

The hospital confirmed the pending surgery but declined to give details until today. Doctors have said that one or both of the boys may die and if they survive, some brain damage is possible. They each have their own brains, but share an extensive attachment of blood vessels.

Ahmed and Mohamed Ibrahim arrived in June 2002 in Texas. They were born a year earlier by Caesarean section to Ibrahim Mohammed Ibrahim and his wife, Sabah Abu el-Wafa, in the southern Egyptian town of Qus.

Dallas-based World Craniofacial Foundation, a nonprofit group that helps children with deformities of the head and face, arranged for the twins to travel to Dallas after an Egyptian doctor put out an international call for help.

Mohamed and Ahmed smile and giggle, babble in English and Arabic and try to move around any way they can. But experts have said they were getting behind in their development, compared with other children their age, because they were unable to explore their world.

Father Ibrahim Mohammed Ibrahim playfully bites on a stuffed toy while he tries to amuse his conjoined twin sons, from left, Ahmed and Mohamed Ibrahim, in April in Dallas. The 2-year-old twins from Egypt joined at the top of their heads will undergo separation surgery today at Children's Medical Center of Dallas.

When doctors concluded that a separation surgery was possible, the risks were explained to the parents. The boys’ father told the doctors to go ahead, saying he wanted to give them a chance at a normal life.

“If they’re left this way, they’re not going to be normal,” Ibrahim said through a translator earlier this year.

Ibrahim spent much of the past year in Dallas with the boys. His wife had remained in Egypt with their two other children but returned to Dallas for the surgery.

Surgeons across the globe have conducted at least five surgeries in the last three years to separate twins joined at the head. Three were successful; one resulted in one twin dying and during another — involving 29-year-old twins from Iran — both died.