This is the fifth such incident in the city over the last three months.
Passers-by noticed the male foetus in Khotwadi area and informed the cops, who took it to Cooper hospital, police added.
"An FIR was registered against unidentified woman under IPC section 318 (concealment of birth by secret disposal of dead body). The five-to-six-month old foetus was sent for a post mortem and search for its parents is on," said Arun Chavan, senior inspector at Santacruz.
On June 9, a five-month old foetus was found in a drain in Kandivali. A day before this, two foetuses were found in a garbage bin near a civic school in Kurla.
On April 14 this year, the Cuffe Parade police had found a female foetus on a playground located near Backbay bus depot. The remains, which were partly buried, surfaced while children were playing there.