RIO DE JANEIRO, Feb. 20 (Xinhua) -- Brazilian doctors have successfully carried out the first operation to separate a pair of Siamese twins joined at the head in a complicated procedure that will continue till November, TV Bandeirantes reported Tuesday.

The operation was done Saturday at the medical hospital of the University of Sao Paulo in Ribeirao Preto municipality near Sao Paulo with the support of the Montefiore Medical Center in New York.

Overseen by neurosurgeon Helio Rubens Machado, it was the first of the four operations that Maria Ysabelle and Maria Ysadora, born in 2016, underwent. They are expected to be completely separated by November.

American surgeon James Goodrich, a pioneer in such operations, was on hand to assist.

Before the surgery, the skulls and the brains of the girls were reconstructed as a 3D model.

The procedure to disconnect the blood vessels and expand the skin of the head lasted seven hours.

"Everything we had planned has gone well," Dr. Machado said.

Over the next three surgeries, a part of the skull will be opened and the veins and the superposed areas of the brains will be separated. The twins will have to spend several months in recovery between each operation, with the final intervention expected to be the most complex.

Cases of Siamese twins joined at the head are very rare. Operations to separate them are very expensive, estimated at around 2.5 million U.S. dollars in the U.S. healthcare system.