I'm not going to tip a poor pregnant waitress
Why doesn't she deserve a tip?
I wouldn't tip anyone while they are about to give birth.
That is no reason to not give a tip. If she is your waitress, she should get a tip.
I do tip employees during holidays or when they have difficult jobs.
Unless she was very rude or dropped food on you, not tipping her is incorrect. Especially since she is pregnant, she could use the money.