The 26-year-old expectant mother had spotted an advert for baby clothes on Craigslist, so arranged to meet with the seller at her house in Colorado to pick them up.
When she arrived, however, she was viciously attacked.
Police Commander Jeff Satur told local press: "When she got here, she was assaulted and stabbed, and her baby was removed.”
The young mother-to-be was rushed to hospital, where she was treated for the stab wounds to the stomach.
She is expected to recover. However, her baby did not survive the ordeal.
Police believe the suspect, 34, is mentally ill, as she arrived at the hospital with the baby and claimed to have suffered a miscarriage, Satur said.
She has been arrested on suspicion of attempted murder, assault, and child abuse.
However, due to complicated laws regarding the lives of unborn children, it remains to be seen what formal charges will be filed against her.
Boulder County District Attorney Stan Garnett explained: "The issues involving an unborn child are complicated under Colorado law.
"In most circumstances, if a child was not actually born alive, then homicide charges are not possible."
This story has once again highlighted the potential dangers of online classified websites.
As a result, a growing number of police stations have started offering "safe zones" for online sellers and buyers to conduct their transactions after recent deaths linked to the likes of Craigslist.