"Sure, my DD isn't here, yet, but that doesn't make me any less of a mother."
Actually, it does. I am with your co-workers. I wouldn't call myself a mother until I have a child. This is just the transitional phase.
I don't mean to trivialise your feelings but I just want to make the point that it really doesn't seem a hurtful thing to say to me, so assuming we think the same, your co-workers didn't mean to hurt you and probably don't understand why you would be upset.
I have to agree with this, and disagree with most of the previous posts on this thread.
My DW is expecting in July and we didn't celebrate Mother's Day. As we see it, we are not parents
yet - and won't be until she's holding that little bundle of love in her arms.
That's our view. It's also the view of our families and friends. Obviously, OP, your co-workers share that view, too.
In no sense is that intended to minimize what expecting mothers-to-be go through (though our families and friends would, I'm sure, be quick to remind us that We Ain't Seen Nothin' Yet). Nor is it some sort of personal or generalized attack on, or disparagement of, you and your condition. Of course no-one can stop you from celebrating it, but other people will legitimately hold the view that Mother's Day is for people who have already had children, not for expectant mothers. You cannot expect to bend everyone's opinion to match yours and it's useless to get oversensitive because people disagree with you. Does it occur to you, OP, that from their perspective, it might look as if you're being just a little bit self-centred and wanting everything NOW?
If you crave their acceptance, then you'll have to be patient. Your time will come. It's not for much longer, after all.
But of course, you don't exactly need their permission or acceptance anyway. Do what you want.