The accused raped the girl around a year ago and repeatedly thereafter, police said.
The matter came to light yesterday, when the victim was rushed to a hospital in Kalwa after she started feeling uneasy and vomited, they said.
"Doctors who examined the girl concluded that she was seven months pregnant, based on which we slapped charges of rape on the accused," police said, adding that due to social issues, they would not reveal any names.
The accused had threatened the victim with dire consequences if she dared to reveal anything to anyone, they said.
The arrested youth, who was unemployed, had twin sisters and all of them lived with their parents, police said.
One of the sisters used to go to work along with her parents, while the other, the victim, used to stay home, they said.