I think it depends on the manager. By right there shouldn't be any discrimination based on pregnancy, gender, race, sexual orientation, religion etc. But like you said if an employer wants to fire you or hold your promotion because of any of these reasons, they can always mask it up with some other reasons. But if you have a great track record and your performance was not affected by pregnancy, no harm in questioning back why you are not given the promotion. If you aren't happy with the reply you can seek help from HR stating you doubt your pregnancy could be the reason. I know of a friend whose lady manager gave her low rating on her annual performance because she was taking pumping breaks which her other colleagues weren't taking. Duh! 🙄