A two-year-old boy also suffered serious head injuries in the crash in Birmingham.
The driver of a black Vauxhall Astra ran from the scene in Eastfield Road, Bordesley Green, after crashing with a red Ford Fiesta.
The driver of the Fiesta was eight weeks pregnant. Another child, a four-year-old boy, suffered minor injuries.
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PC John Slater, from West Midland Police's traffic collision investigation unit, said: "Tragically the woman was in the early stages of pregnancy at the time and as a result of the collision suffered a miscarriage.
"I would like to hear form anyone who may have seen the crash to call police on 101.
"I would also ask that the driver does the right thing and makes contact with the police."
Police said the crash happened on Saturday 2 April at 15.25 BST.
A hit-and-run driver is being hunted by police after a crash caused a pregnant woman to suffer a miscarriage.