A risky operation to separate Romanian twins joined at the head was stopped after complications made it dangerous for doctors to continue, the Cleveland Plain Dealer newspaper reported Thursday, according to dpa. Doctors at the Rainbow Babies and Children's Hospital in Cleveland, Ohio broke off the operation after 12 hours because of low blood pressure and swelling in the brain of one of the three-year-old sisters, Dr Nathan Levitan said. Tatiana and Anastasia Dogaru share a circulatory system and some brain matter. The team of 50 neurosurgeons working on the separation stopped the surgery because they felt it was risky to continue without knowing the cause of the complications. Further tests were planned. Operations to separate twins conjoined at the head have been performed only a handful of times and carry a very high risk. Without the operation the girls would likely not live beyond their 10th birthday. THe children of Alin and Claudia Dogaru were born in January 2004 near Rome and came to the US in late 2004 to seek medical treatment.