As noted by this CBC report, an Arizona judge has ruled that a fetus doesn't count as a passenger in the car pool lane. An Arizona woman was ticketed for driving alone in an HOV lane, but fought it in court on the grounds she was pregnant at the time.
Of course if she had won, this would have opened a whole new range of legal questions (never mind the complications it would present to Roe v. Wade). If expectant mothers were to be allowed to drive alone in the car pool lane, surely expectant fathers would have to have the same right. Otherwise, that would be sexual discriminaton! :)