According to detective Azuen Magda Albarello, Carmo confessed to killing the married mother who worked as a beautician.
Carmo revealed she found her victim online after the mother-to-be posted an appeal for help for clothes for her baby.
Detective Albarello said: "Suelen joined a mother's group on social network and saw Naiara was asking for help.
"They began to exchange messages, and the woman agreed with the victim to meet in Goiania, pretending she would deliver a complete trousseau for the baby.
"Suelen collected Naiara in the capital, then took her by car to her house in Neropolis (a neighbourhood in the city) where the crime happened."
The accused is reported to have had an abortion two months ago after splitting from her partner who allegedly didn't want the child.
Neighbours who were not aware of the accused circumstances said she 'wore large flowing dresses and looked like she was pregnant."
Cops were alerted after a suspicious neighbour, who was asked to dig the hole in the yard for Carmo, reported his concerns.
Marcos Pereira, a bricklayer, said: "Suelen called me to ask if I could do her a favour because she wanted to plant some spring onions in her garden, but as she was pregnant it would be difficult. I said no problem.
"I dug down far enough to plant vegetables but she insisted on the hole being a metre deep by two metres wide. I found that strange and unnecessary.
"Then when she said she was going to get some cement to put around the hole to make it look nice, I became really suspicious because she said she was pregnant but she didn't need any help carrying the cement bag."
When officers arrived at the property they described the accused as nervous and unwilling to open the door to answer any questions.
A cop jumped over the property wall to inspect the area and discovered the victim's mutilated body buried in the makeshift grave, partially covered with dirt.
Detective Albarello said under questioning Carmo gave details of the cold-bloodied killing and revealed the young mother suffered a horrific death.
She said: "Naiara died in a terrifying and helpless way probably knowing her child was going to be taken from her. She was doped with a powerful dose of rivotril, a lethal drug that incapacitates the victim.
"Once the medication took hold the accused confessed she tied a rope around the victim's neck and hung her until she was sure Naiara was dead.
"Carmo told us, at that time, the baby was still alive and moving in the woman's womb.
"She took a scalpel and slashed open the pregnant belly. But she was shocked to find the baby was in a bad way because the umbilical cord was wrapped twice around the child's neck.
"Carmo tried to breathe life into the infant but all her attempts to resuscitate failed," Detective Albarello reported.
Police found plastic gloves, a scalpel and the medicine apparently used to drug the victim in the property.
Detective Albarello said: "The evidence indicates that this heinous crime was pre-planned and we believe Carmo was intent on getting a baby for herself after the loss of her own child."
Costa's husband, who has not been named, is said to be under sedation following the shocking murder of his wife and child.
Carmo is facing two charges of murder and has been remanded in custody to appear in court next month.
