Getting fired during pregnancy???

Hey ladies! Has anyone ever been fired after telling their employers you're pregnant? I ask because things at my work have been tense and a little hostile since I told them I am pregnant. I told my boss early, so it would not affect other people's vacation time since I am due in July. Since then, my boss and her boss have asked me almost every week if I am coming back after my maternity leave. I feel like they're trying to trick me into saying I won't come back so they can fire me. Are they allowed to ask me this so often? 
I even heard my supervisor and other employee discussing it (we have cubicles so it's not like it's hard to hear everything) and saying that it's nonsense I'm still allowed to be here since I am just leaving anyway. 
For the record, I 100% need my job after giving birth, so I don't have any plans on leaving.
Have you guys ever heard of an employer coming up with a reason to fire someone while they're pregnant? This is crazy since I work in a place where literally 99% of us are women in our 20s and 30s.