The baby died. The woman, 26, is expected to survive.
The assault happened around 2:45pm on Wednesday (local time) in Longmont, a Denver suburb, said Cmdr. Jeff Satur, a Longmont Police Department spokesman.
Police believe a 34-year-old woman lured the victim to her home with the online ad for baby clothes, Satur said.
The assailant attacked the woman and immediately fled the residence with the baby, police said. The victim, left alone in the home, called 911 and was taken to Longmont United Hospital, where she underwent surgery.
Police found the suspect, named by Reuters as Catrece Dynel Lane, at the same hospital. She had taken the baby there, claiming she had a miscarriage, Satur said.
Police would not say how far along the victim was in her pregnancy.
Lane was arrested on suspicion of attempted first-degree murder, first-degree assault and child abuse knowingly and recklessly resulting in death, police said.
