Led by a Los Angles-based neurosurgeon, the medical team planned to spend about 13 hours removing Rebeca Martinez's second head, which has a partially formed brain, ears, eyes and lips.
Eighteen surgeons, nurses and doctors were to take several rotations to cut off the undeveloped tissue, clip the veins and arteries and close the skull of the 7-week-old girl using a bone graft from another part of her body.
"The head on top is growing faster than the lower one," said Dr. Jorge Lazareff, director of pediatric neurosurgery at the University of California at Los Angeles' Mattel Children's Hospital. "If we don't operate, the child would barely be able to lift her head at 3 months old."
Lazareff said the pressure from the second head, attached on top of the first and facing up, would prevent Rebeca's brain from developing.
The operation's start was delayed for about four hours due to complications in administering anesthesia.
"The girl is stable. So far all her vital signs are fine," said Dr. Santiago Hazim, medical director at the Center for Orthopedic Specialties in Santo Domingo, where the surgery was being performed.
CURE International, a Lemoyne, Pennsylvania-based charity that gives medical care to disabled children in developing countries, is paying for the surgery, estimated at $100,000. The agency funds the Center for Orthopedic Specialties.
The operation is risky because the two heads share arteries.
"When the doctors come out and tell us it's all OK we'll be filled with happiness," father Franklin Martinez, 29, told The Associated Press Thursday.
Lazareff was to lead the operation along with Dr. Benjamin Rivera, a neurosurgeon at the Medical Center of Santo Domingo and the Center for Orthopedic Specialties. Lazareff led a team that successfully separated conjoined Guatemalan twin girls in 2002.
Doctors say if the surgery goes well she won't need physical therapy and will develop as a normal child.
Twins are born conjoined at the head when an embryo splits to make identical twins and then stops growing, leaving them fused. Such twins are rare, accounting for one of every 2.5 million births.
Parasitic twins like Rebeca are even more rare. They occur when one stops developing, leaving a smaller, partially formed twin dependent on the other.
Rebeca is the eighth documented case in the world of craniopagus parasiticus, Hazim said.
All the other documented infants died before birth, making it the first known surgery of its particular kind, according to Lazareff and the other doctors.
Martinez and his 26-year-old wife, Maria Gisela Hiciano, say doctors told them Rebeca would be born with a tumor on her head but none of the prenatal tests showed a second head developing.
Although the second head is only partially developed, its mouth moves when Rebeca is being breast-fed.
Martinez works at a tailor's shop. Hiciano is a supermarket cashier. Together they make about $200 a month. They have two other children, ages 4 and 1.