A Colorado woman has been arrested for attempted first-degree murder after stabbing a pregnant woman and cutting her baby from the womb.

The 26-year-old victim, who was seven months pregnant, went round to the stranger's house in Longmont after responding to an advert on Craigslist in the hope of buying some baby clothes when she was attacked at around 2.30pm on 18 March.

"Officers arrived on scene and could hear a female calling for help inside the home. Officers entered the home and located a female who had been beaten and stabbed in the stomach," a statement from police said, according to The Longmont Times-Call.

"It was later learned, the female was pregnant and her baby had been removed. The victim was transported to the Longmont United Hospital where she underwent surgery and is expected to recover."

The 34-year-old suspect was found with the dead baby at the hospital and told staff the victim had a miscarriage. She was later arrested for attempted first-degree murder, first-degree assault and child abuse knowingly/reckless resulting in death. The two women were not known to each the other.