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'Teen Mom' star Amber Portwood has been arrested for domestic battery ... TMZ has learned.

She was arrested in Indianapolis Friday just before 6:00 AM. Cops are not saying who the alleged victim is ... per protocol.

Portwood was still in custody at the time of this post.

The 29-year-old and boyfriend Andrew Glennon just had a pregnancy scare on the "Teen Mom OG" show. She also hinted they might soon get married.

She has 2 kids -- 1-year-old James, whose father is Andrew, and 10-year-old Leah, whose father is her ex-boyfriend Gary Shirley.

Portwood went to prison for 17 months back in 2012 for violation her probation in a drug possession case. She actually had a choice of prison vs. rehab ... she chose prison.