A US woman who has been in a coma for five months has woken up – and discovered that she has a baby son.

The family of Sharista Giles were told that the 20-year-old might never wake up after a car crash last December, when she was five months pregnant.

But the Sweetwater, Tennessee family never gave up hope and in January "Baby L" (the family is yet to name the child) was delivered at the Harriman Care and Rehabilitation Centre.

Though he weighed less than 900 grams when he was born Baby L has gained weight steadily and now weighs almost 3kg.

The family moved Ms Giles to the centre in March, and descended en masse when the news came that she had opened her eyes on Wednesday.

(Facebook)

"The whole place was packed out with family," Beverly Giles, a relative, told ABC News.

At the time of the accident the family was told Ms Giles had a 10 percent chance of survival at best and though the extent of the damage isn't yet known, she is responsive, squeezing hands and following Baby L's father around with her eyes.

"He showed her a picture of her baby, and she followed the picture," Ms Giles said.

"When he turned around to put it back on the bulletin board, she turned her neck, her whole head trying to follow and find the picture again."

The family says they will bring Baby L to meet his mother as soon as he is ready to leave the neonatal care unit at the University of Tennessee Medical Centre.