I don't think it's a question of "rights" but a question of what the actual practice in question dictates. Literally the point of HOV lanes is
"The goal of HOV lanes is to provide an incentive to use ridesharing".
High-Occupancy Vehicle Lanes | US Department of Transportation
A woman pregnant with triplets is no more "ridesharing" than any individual dude going down the road.
If we were to really run with a pregnant person taking up actionable occupancy why not require an extra full ticket for an unborn on a plane, train or bus? To see a movie? Sporting events? I don't think this argument is particularly useful when trying to parse rights vs taking up physical space as separate entities.