The girl was found in the jungle in April by laborers working on a natural gas pipeline from the Coari oil fields to Manaus, a city of 1.5 million people 1,800 miles (2,900 kilometers) northwest of Rio de Janeiro, said the bureau, known as Funai.
She was taken to Manaus and went into labor on Thursday. The baby, weighing 2.2 kilograms (4.9 pounds), was delivered by Caesarean section and reportedly was in good health, said Daniele Santos of the Funai press office.
The mother, of the Apurina tribe, has refused to speak, and the identity of the infant's father was unknown, Santos said.
"In some cultures it's natural for girls to have sexual relations after their first menstruation, but now we have to study if that was the case here, if their culture permits this," Santos said.
Mother and child were to remain under medical care for a month and then return to their village in Jaturana, about 75 miles (120 kms) from Manaus, she said.
"It's very rare, and we have to investigate whether it was rape," said Marina Mota, a press officer for the National Health Foundation. "The tribe wasn't that isolated, and had a lot of contact" with outside cultures.
National census figures show Brazil has about 700,000 Indians. Most of them live in the northern Amazon rain forest, reports AP.
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