Nguyen Thi Thuy, 23, was detained by Dong Thap police on Wednesday to be investigated for murdering her newborn child.
Thuy said her 14-day-old child had been kidnapped while he was sleeping in a hammock on Tuesday.
Cao Lanh city police launched an investigation and found a plastic box containing the child's dead body floating along a river in My Tan Commune on Wednesday.
Investigators said they noticed several abnormalities in the case, as well as inconsistencies in Thuy’s story, so they probed further.
Thuy eventually admitted to having put the child in the box before letting it float down the river.
She said she had suspected that her husband had been unfaithful, and following a verbal altercation with him, decided to do what she did.