Police were called about 6 p.m. to a home in the 1300 block of South B Street in San Mateo, where the 37-year-old woman and her unborn child had died. She was 9 months pregnant, police said.
Authorities haven't released how the woman died, but said that family members found her at the home, which she shared with the suspect.
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Witnesses and evidence at the scene led police to suspect the woman's boyfriend, Andrew Coleman, and detectives used technology to track him as he was driving toward Southern California, police said.
San Mateo investigators requested assistance from other law enforcement agencies, including the California Highway Patrol and the Los Angeles Police Department.
The suspect's car was eventually located by the West Covina Police Department and he was taken into custody there on Sunday night.