A man who raped and impregnated his 11-year-old granddaughter has been handed a jail sentence of 200 years.

Mikeal Shane Pruett, 55, of Stevensville, Montana, was also found guilty of sexually abusing the girl's younger sister.

He was given a 100 year sentence for each offence. He will be eligible for parole when he is 105.

Ravalli County District Judge, Jeffrey Langton said that Pruett had a history of sexually abusing children.

"You seem to lack any semblance of a conscience," Langton said, as reported by the Missoulian.

Pruett was arrested in June 2014, after his granddaughter visited a doctor and was found to be pregnant.

She initially told authorities that she had become pregnant after meeting a boy at a county fair, but after she gave birth she said the father was Pruett, who had concocted her prior account.

She said she didn't know she was pregnant until a doctor diagnosed her at 32 weeks. She gave birth in August 2014.

"I didn't know what pregnant felt like. So I didn't know," she told child advocates.

Pruett had twice before been charged with sexual offences.

"In my view, over the course of his life, he has set no boundaries when it comes to committing sexual offences. Everything about the defendant's life and conduct in Ravalli County has earned him a 100-year sentence," said Ravali County Attorney Bill Fulbright.