Just like everyone else, a pregnant woman can be ticketed for driving in the carpool lane without anyone in her passenger seat. The carpool law requires that multiple seats in the car contain passengers, according to Berkeley Patch.
The purpose of the carpool lane law is to remove excess traffic from the roads by encouraging people to share cars, taking up more physical space in the vehicles. In 1987, a woman was ticketed for driving alone in the carpool lane in California. Convinced that her unborn child counted as another person, she took the case to court, resulting in the dismissal of the ticket, according to the LA Times. A later case in Arizona was upheld, however, according to CBS News.