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The man, 55, was told by a judge he lacked "any semblance of a conscience" as he was found guilty of two counts of incest.

After impregnating his granddaughter he told her to tell doctors the father was a 14-year-old boy with whom she had slept with.

But medical staff were sceptical of her claims and continued to question her.

During a later interview she admitted her grandfather had forced her to make up the story.

The man, who cannot be identified for legal reasons, was arrested in June after his victim was found to be 32-weeks pregnant.

She also said he had assaulted her by "kissing her privates".

The girl had no idea she was carrying a baby as she "didn't know what pregnant felt like".

The grandfather also sexually assaulted the girl's younger sister.

The man, from Stevensville, Montana, pleaded guilty and briefly apologised to his victims.

He will have to serve 50 years in prison before being eligible for parole.