Police had already been looking for the baby, whose body was found dead in a nature reserve in Willebroek, in the Antwerp province, on Saturday, for two weeks.
The mother of the baby, who has been identified as an underage girl, reportedly told staff members of a hospital in Willebroek two weeks ago that she dumped her daughter in one of the three ponds in the Blaasveldbroek nature reserve.
The staff then informed the police, according to reports from De Standaard. This resulted in divers and officers looking for the body in the nature reserve.
Finally, a hiker found the baby on Saturday and alerted the police. The public prosecutor's office, the forensic laboratory and a medical doctor arrived on the scene, and half the nature reserve was closed off by the police, who at the time said it immediately opened an investigation into the suspicious death.
The mother, who is reportedly receiving counselling following the incident, said the baby was dead when she was thrown in the water, a statement which will be investigated as part of the legal investigation.
The autopsy report's results, which are expected later on Monday, should provide more clarity on the cause of death. According to preliminary findings of the lab and the medical officer, the baby was fully-grown, which would rule out a miscarriage.
The punishment faced by the mother will be decided on by a juvenile court judge. She may be handed over and tried as an adult, especially if the autopsy report can verify that the child was still alive when she was thrown into the water, as that would constitute a murder case.
Earlier this year, the body of another newborn baby was found in a park in the Netherlands just across the border with Belgium.