The pregnant woman, identified as Shakila Imran Siddiqui, was brought to CSM Hospital with foetus hanging out from her abdomen, Thane civic officials said.
The remaining victims are yet to be identified, they said.
The building collapsed at around 6:30 PM, the officials said, adding, the injured were rushed to various hospitals.
Civic officials said the building in Shil Dhaigar locality was unauthorised. They claimed that it was built on forest land and was occupied at least up to the fourth floor.
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Rescue and relief operations continued till late in the night as victims were feared buried under the debris.
More than a dozen fire tenders along with over 24 ambulances were stationed at the site of the accident.
The operations were carried out under the supervision of Civic Commissioner R A Rajeev and District Collector P Velarasu.
Additional Collector Ashok Shingare took charge of the district disaster control room and made arrangements for the civil defence and other rescue teams.
The injured were admitted to various hospitals including Thane Civil, CSM Civic hospital at Kalwa, Kalsekar hospital in Kalwa, while those in critical condition were shifted to Sion and JJ Hospital.