BOSTON — Sandra Birchmore’s former colleagues described her hopes for her pregnancy Tuesday as testimony began in the federal murder trial of Matthew Farwell, a former Stoughton police detective accused of killing her in 2021. Cheryl Carlson, a former assistant principal at the Sharon elementary school where Birchmore worked as a teacher’s aide, told jurors Birchmore had said she was pregnant. The reported suicide ruling “didn’t make sense,” Carlson testified, because Birchmore was “so excited to have that baby.” Birchmore, 23, was eight to 10 weeks pregnant when she died at her Canton apartment in February 2021. Prosecutors allege Farwell, now 40, strangled her and staged her death to appear to be a suicide. They also allege he had sexually abused her for years, beginning when she was a child, and say Birchmore had told him he was the baby’s father. The prosecution’s first witness, Stoughton police Detective Heather Cheever, testified that she met Birchmore when Birchmore was 15 and participating in the department’s youth explorer program. Cheever described herself as a mentor and said Birchmore talked with her about family and school, but did not tell her about alleged one-on-one tutoring by Farwell or sexual contact between them. Prosecutors showed jurors video they said depicts Farwell entering Birchmore’s apartment building on the night she died. The person shown wears a hoodie and face mask. Cheever testified Farwell did not wear a mask at the police department, even during the COVID-19 pandemic. “He didn’t care about COVID or wearing a mask,” she said. Cheever also testified that she would have reported a relationship between Farwell and Birchmore had she known about it. Defense attorneys questioned Cheever about the closeness of her relationship with Birchmore and Birchmore’s mental health, including a 2020 hospital visit after statements about self-harm. Farwell has pleaded not guilty. His defense points to the state medical examiner’s initial finding that Birchmore died by suicide. The medical examiner withdrew that opinion in April, and the cause of death is now listed as undetermined. To convict Farwell of Birchmore’s death, prosecutors must prove he killed her to prevent her from reporting that he had committed a crime. He is also charged with causing the death of her fetus. If convicted, he faces life in prison. Farwell has been held without bail since his arrest by the FBI on Aug. 28, 2024. The testimony opened a trial centered on Birchmore’s death and the conduct of a police officer who prosecutors say abused the trust and authority he held in her life. A manager of Birchmore’s apartment building also testified Tuesday about Farwell appearing on security footage, according to courtroom updates.