Member Rara Avis
I think you have received very good advice from your passionate friends, here. I especially support the idea of documenting what has happened and what was said by the owner of the company and in presenting the facts to the public legal system in your jurisdiction for evaluation, if need be.
What I would do is this: (keep in mind, this is just my opinion and I'm not an expert in legal advice)
First, as documentatoin, draft a letter to your direct supervisor with a carbon copy to the owner of the company. Within that letter, state the facts first.... how you were accustomed to being on the schedule for X amount of hours per week and how your work time has been decreased to 15 hours per week. Then, make reference to the Christmas party and how the owner of the company answered you when you asked him about your reduced hours (quoting him would be good). Then, close the letter by stating something like, "I will be happy to provide the company a written statement from my doctor to the effect that I am physically able to work X # of hours, just as I was prior to becoming pregnant. Please let me know whether this is necessary." If I was writing the letter, I might also make a statement at the end which questions whether you are being discriminated against because you are pregnant (but, i'm often bolder than a lot of people).
Clearly, you are in a difficult situation and I would encourage you to be assertive, yet also cautious in your wording. If you have a public resource in your area for legal assistance, they may be able to help you draft this letter.
Again, all of this is just my opinion... I am not an expert on how to handle things like this which may have some legal implications. To be honest, I'm very surprised that you are experiencing this blatantly politically incorrect scenerio in 1999, as we approach 2000. I thought the world knew better. I have 2 children... my daughter is 7 and my son is 17. With both children, I worked up until the day my water broke. Unless you have complications, there is no reason you can't continue working throughout your pregnancy.
Hang in there, Lady! And keep us posted, ok?