You have my total sympathy. I have never been pregnant, or had a child - but I have experienced the same frustration at having a ridiculously increased workload, while the person with the baby-on-the-way or 'health issues' props their feet up, and takes more liberties than I could ever imagine was reasonable. And don't get me started on the co-workers with kids who thought that, because I don't have kids, I should obviously take all the holiday shifts.
All of this being said - the ladies on this forum are right - there is a very real human issue, and I have never liked to rock the boat, because I feel there is very real discrimination against women in the workplace, because they have babies, or the ability to have babies - and I think that underlying discrimination keeps many deserving women out of higher-paid positions.
Since you already tried talking to your supervisor, and really are getting nowhere, I would sit down and re-think all of this. If you complain further, are you going to be branded a troublemaker? (Sad fact, but true). Can you vent here, and feel a little more at peace with the situation, which, in the end, isn't a forever-thing? If you are still really bothered by this, I would I would request an appointment with someone in your human resources office, and talk to them about the situation.
At the very least, if your co-worker is going to get to sit down and have a meal on the company clock, everyone in your office should be afforded the same privilege.