She confessed to getting the victim drunk and tying her to a tree with metal wire before mutilating her womb
Police arrested Angelina Rodrigues, 40, who, according to reports, "always dreamed of having a baby girl".
She allegedly arrived at Sao Lucas hospital telling doctors that she had just given birth. Doctors became suspicious and called police when she refused medical checks.
Her husband Roberto Gomes da Silva was also arrested after Mara's body was found. She confessed to getting the victim drunk and tying her to a tree with metal wire before mutilating her womb.
Police arrested Roberto doubting she could have done it on her own - despite her denying he was there.
A police spokesman said: "We believe that the husband and a third person took part, but all this will depend on a police investigation.
"Tests show that Angelina isn't pregnant and we believe the crime was premeditated."
Family members said twisted Rodrigues befriended the victim after learning she was pregnant, cooking up her sick plan.
Her aunt Euza Ribeiro told local media: "The woman always dreamed of having a baby girl, and when she saw that my niece was pregnant she began to offer help.
"She lent her money to have her ultra-sound scan and the day she died had told her she was going to buy all the things that the baby needed."