An Indonesian woman allegedly drowned her nine-year-old son in the bathtub because she was worried his small penis would harm his future prospects, AFP reports.

"She told police investigators that she killed him as he would have a bleak future with his small penis," Jakarta police spokesman Rikwanto, who goes by one name, told AFP.

"She drowned her son in a bathtub filled with water. She then dressed him and laid him on a bed. After that, she went to a nearby police office to report her crime."

The woman, 38, reportedly told Jakarta police the boy always had a small penis, but after he got circumcised, it appeared even smaller.

Rikwanto said he believes the woman knew exactly what she was doing, but police have ordered a psychiatric evaluation to assess her mental condition.