Former Stoughton police detective Matthew Farwell. Uncredited/Associated Press

A former Stoughton police detective accused of killing a young pregnant woman whom he allegedly began sexually exploiting when she was 15 years old now faces prosecution for the death of the male fetus she was carrying.

In a superseding indictment filed Tuesday, a federal grand jury added the charge against Matthew Farwell, 39, who is accused of killing Sandra Birchmore in her Canton apartment on Feb. 1, 2021.

The indictment marks the first time authorities have identified the gender of the unborn baby she was carrying. Birchmore was eight to 10 weeks pregnant when she died, the indictment said.

DNA testing has excluded Farwell as the father of her unborn baby , and it’s unclear whether federal inves

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