A woman is in a critical but stable condition after an attacker cut her unborn baby out of her womb, her family said.

Michelle Wilkins, from Longmont, Colorado, was reportedly attacked after answering an online ad for baby clothes.

In a statement, the family of the 26-year-old thanked police for their work and doctors and hospital staff for providing compassionate care since the attack on Wednesday.

Dynel Lane, a mother of two, is accused of stabbing Ms Wilkins, removing the baby girl and taking the dead foetus to the hospital, claiming she had had a miscarriage.

Lane is suspected of luring Ms Wilkins to her home with an ad on the Craigslist website.

Wilkins, who was nearly eight months pregnant, managed to call 911 for help.

Authorities said 34-year-old Lane told her family she was pregnant.

Lane's husband told investigators that when he came home from work early to meet her for a pre-natal appointment, he found her covered in blood and a baby gasping for breath in a bath.

Lane told her husband she had suffered a miscarriage, and he took her and the baby to hospital, where she was later arrested on suspicion of attempted first-degree murder and other crimes.

While drifting in and out of consciousness, Wilkins told police she did not know Lane and only went to her house in response to the ad, an affidavit says.

Lane went to great lengths to show her family she was pregnant, sharing an ultrasound photo with her daughter, the affidavit says.

She and her former husband lost a 19-month-old boy in a drowning accident in 2002.