The incident came to light yesterday after the girl was diagnosed to be two months pregnant.
On being questioned, the girl broke down and narrated her ordeal to her mother, said police.
The girl's mother, who works as a housemaid, along with a relative approached the police and reported the matter, following which the man was arrested, said police.
The accused has been booked under relevant sections of the IPC and POCSO Acts, and the girl was counselled by an NGO, police said.
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But the period over which she was allegedly assaulted could not be ascertained yet, said police.
The girl's father passed away around seven years ago after which her mother married the accused.
The girl dropped out of school around a year after that, police added.