Yes; this may well be a "pretext" firing--that happens when they are really firing you because you're pregnant, but they claim that they are firing you because you made a mistake, stole a ball point pen, arrived 1 minute late, etc.
Call an employment attorney ASAP (tomorrow) and be careful about talking to anyone else until you contact an attorney.
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I'm sorry you're dealing with this stress, especially during your pregnancy.
As an at-will employee, you can be fired for any reason, or no reason at all, as long as you're not being fired for an illegal reason, such as being pregnant. That being said, if they're trying to fire you for another reason, such as being late, but the real reason behind the termination is your pregnancy, it is a 'pretext' as the other attorney answer suggests. However, from the very limited facts you've provided here there is no way to answer your question.
I would highly recommend you seek a consultation with an employment attorney in your area as soon as possible. Many offer free consultations, so you may want to start there. A consultation is the best way to get an idea of your rights and whether they were potentially violated. Try to seek a consultation as soon as possible, as there are time limits involved with situations such as this. Good luck moving forward.
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