Korena Roberts, 27, battered eight months pregnant Heather Snively with a collapsible police baton after luring her to her home in Oregon, Daily Mail reported on Thursday.
She then used a sharp instrument to cut the unborn baby from Snively's abdomen.
An autopsy showed that the baby boy, who did not survive, had been torn from Snively's womb.
Her body was found stuffed into the crawlspace in the kitchen of Roberts' home in Portland, Oregon, in June 2009.
Police were alerted to the murder after paramedics were called to the home and discovered Roberts' boyfriend Yan Shubin desperately trying to revive a newborn baby and large amounts of blood on the floor.
Roberts, who has two children under 10, told paramedics she was the mother of the baby and was rushed to hospital, where an examination revealed she had not given birth.
For months before the killing, Roberts told neighbours that she was pregnant, going as far as buying a stroller, baby formula and parenting magazines.
Weeks before her death, Snively moved to Oregon from Maryland to be with her fiance.
Detectives said they believe Roberts and her victim met online through the Craiglist website where both women were looking for baby clothes.
During a bail hearing, Roberts' lawyer said she had given birth to a stillborn child in 2007 and was obsessed with babies, repeatedly watching videos of births on YouTube, sewing baby clothes and telling people she was pregnant.
Neighbours said they often saw Roberts walking with Snively.
Detectives said they believed Roberts deliberately lured Snively to her home with the intention of killing her and stealing her unborn baby.
Snively's mother Heidi Kidd said Roberts told her daughter she was pregnant and wanted to exchange baby clothes.
Kidd, who earlier revealed the baby boy would have been called John Steven, said: "I guess they decided to meet and I guess that was the wrong thing to do."