A motorist spotted the muntjac lying beside the road in Cambridgeshire and decided to help.
He put the deer in his car boot, drove home and called Fenland Animal Rescue.
A scan found the animal had suffered lacerations and a dislocated ankle but also that she was heavily pregnant. The centre said on Sunday morning the doe had given birth overnight.
Joshua Flanagan from Fenland Animal Rescue said deer were difficult to treat and often had to be put down as human contact causes them too much stress.
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However, vets managed to treat and stabilise the doe and she is recovering well at the rescue centre.
Mr Flanagan said: "We had a specialist veterinary surgeon check the baby's health with great results all around."
The deer was "safe and comfortable in our care and eating us out of house and home", he added.
They had hoped to release the muntjac under licence from Natural England before she gave birth.
However, that did not happen. "A beautiful healthy fawn was born with no complications," Mr Flanagan said.
He said it had been an "amazing decision" on the part of the motorist who picked her up and "a bit of a Christmas miracle" as deer rarely survived this kind of trauma and human intervention.
They hope to release the doe and her fawn in the near future.
Rescuers of a deer injured by a car were amazed to discover the animal was pregnant.