By Tony Hanson

PHILADELPHIA (CBS) — A Philadelphia man has admitted to raping the most vulnerable of victims, a severely mentally and physically disabled 15-year-old girl.

READ MORE: Teen Victim Speaks Out For First Time Following Racially Motivated Attack Aboard SEPTA Train

The defendant, 55-year-old Orrin Thomas, was a friend of the victim’s mother. He pleaded guilty Tuesday to rape and endangering the welfare of the helpless child.

“She suffered from cerebral palsy, she was blind, she was basically confined to a wheelchair,” said Prosecutor Elizabeth Fischer, “and this defendant, while he was caring for this young girl, did impregnate her.

The victim, fed by a tube, is unable to speak, and Fischer says the crime was only discovered as the mother noticed her daughter’s belly getting bigger — and tests confirmed she was pregnant. The pregnancy was terminated, but Fischer says DNA confirmed Thomas was responsible.

READ MORE: 'Georgia's Defense Is Just So Good': CBS Sports’ Houston Nutt Previews SEC And Conference USA Championships

Thomas will be formally sentenced later, but as part of a plea deal, a 4 to 8 year sentence will be imposed, well below what the defense attorney says could have been a mandatory minimum sentence of 10 years in prison.

The District Attorney’s Office released the following statement in regards to accepting a plea deal in this case…

“Because this defendant took full responsibility for his horrific actions, waived his preliminary hearing to spare the victim and her family from having to testify, had no prior contacts with the law and is 56 years old, we believe this sentence which will keep him in prison into his 60s and under supervision until he is almost 70 years old is appropriate. He will also have to register under Megan’s Law for the rest of his life and register four times a year with state police. This plea also spared the victim and her family the pain of reliving her assault by going to trial, a trial which likely would not have taken place for at least another year.”

MORE NEWS: Open For Business: The Merrygold Shop's Carefully Curated Items Bring Especially Merry Feel To Holidays