The Brazilian girl, who was not identified because she is a minor, was found to be four months pregnant after being taken to hospital suffering stomach pains, the news websites G1, pe360graus and the Diario de Pernambuco reported.
She was being cared for by a medical and psychological team in the Maternal-Child Institude in the northern city of Recife, close to her hometown of Alagoinha in Pernambuco state, they said.
"We don't know if she will develop the pregnancy up to the end because of the structure of her body. It is a big risk for her,'' the doctor who confirmed her pregnancy before she was taken to the institute, Jose Severiano Cavalcanti, told the Diario de Pernambuco.
"She doesn't have a pelvis able to support a gestation of twins,'' he said.
Abortion is illegal in Brazil except in cases of rape or if the woman's health is in danger.
The institute in Recife was unable to immediately confirm information about the case.
The Australian, 15 Dec 2008 According to police accounts reported in the media, the 23-year-old stepfather was believed to have abused the girl since she was aged six, paying her one Brazilian real (about 77 Australian cents) for each sexual relation.
He also allegedly abused the girl's physically handicapped 14-year-old sister.
He was arrested on Thursday as he attempted to flee to another region in Brazil, and was being kept in protective custody.