Dorothy Bruns, 44, was found dead in her New York City home and police say pills and a note were found nearby.
Tony Award-winning actress Ruthie Ann Miles was crossing a street in the city's Brooklyn Park Slope neighbourhood on 5 March when she and her four-year-old daughter Abigail were knocked down by Bruns.
Abigail was killed and her mother was taken to hospital, where she lost the baby girl she had been expecting with husband Jonathan Blumenstein.
Bruns was facing a 15-year jail term after being charged with manslaughter, criminally negligent homicide, reckless endangerment and assault.
The 44-year-old had a seizure when she stopped at a red light before the fatal crash, according to authorities.
Prosecutors say doctors had told Bruns not to drive after she had a previous medical problem.
Media reports suggested Bruns had been battling various health problems, including multiple sclerosis, and had got behind the wheel against her doctor's orders.
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Miles is best known for her Tony Award-winning performance as Lady Thiang in the revival of Rodgers and Hammerstein's The King and I.
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She won for best featured actress in a musical in 2015 and had been due to play the same role in London's West End this summer.