The tragedy occurred just after 10 p.m. Saturday as the white Lincoln Town Car was crossing the San Mateo-Hayward bridge in Foster City, the Bay Area News Group reported.
The fire apparently started in the back of the vehicle, but where or how was under investigation, California Highway Patrol officer Art Montiel said.
Five women were trapped in the flames and four other women managed to make it out of the limo with injuries, Montiel said.
The driver, Orville Brown, 46, of San Jose, survived uninjured.
"We don't know what caused the accident yet," the San Francisco Chronicle quoted CHP Officer James Evans as saying. "It wasn't a traffic collision, we do know that, but we are still investigating."
The westbound lanes of the bridge were closed for several hours.
The fatalities, all women in the 30s and 40s, included Neriza Fojas, 31, of Fresno, for whom the bridal shower was being thrown, her sister, Rosyln Bersamin, told the Chronicle Sunday. Fojas had married recently and was to have returned to her native Philippines for another ceremony in June, the newspaper said.
The names of the other fatalities were not available.
Rosita Guardiano, 73, the mother of one survivor, said the women had attended attended a birthday party in Alameda earlier in the evening.
Her daughter, Mary Grace Guardiano, 42, of Alameda, was in intensive care being treated for smoke inhalation, she said.
Rosita Guardiano said another survivor was Nelia Arrellano, 36, of Oakland. The Chronicle said the other survivors were Jasmine Desguia, 34, of San Jose and Amalia Loyola, 48, of San Leandro.
Kultar Singh, owner of Limo Stop Inc., which owned the car that caught fire, said he was "very, very saddened," but wouldn't discuss the incident.