'Big Bro' Star Arrested for Beating Pregnant GF

A former "Big Brother" houseguest was arrested this weekend after allegedly unleashing a ferocious attack on his pregnant girlfriend.

According to the Boston PD, cops responded to the home of Matt McDonald -- who appeared on BB Season 9 -- after his girlfriend showed up at a local police station to file a complaint.

Cops say Matt's GF -- who's currently 8 weeks pregnant with Matt's baby -- told them the two began arguing after the reality star accused her of cheating on him ... and that's when things got violent.

According to the police report, Matt's GF told cops he began strangling her and yelling, "I hope the baby dies and hope u [sic] have a miscarriage."

The GF claims the next day, Matt accused her of pouring water in his plasma TV -- and then proceeded to kick her in the collarbone, put her in a headlock ... and began punching her in the arms.

During the alleged confrontation, Matt called the victim's father on the phone and she allegedly screamed for help. Matt then screamed, "Shut the f*ck up or I'll punch you in the face." She also claims Matt threatened to kill her father if he called cops.

According to the police report, officers observed visible bruising on the GF's arms, neck and collarbone -- and proceeded to arrest McDonald. According to the police report, McDonald told cops that she attacked him -- and he was simply defending himself.

McDonald was charged with assault and battery -- and assault and battery upon a pregnant person. He's due back in court next month.

As for the baby, we're told the GF is still pregnant.

UPDATE: TMZ has learned the two were engaged to be married at the time of the incident.