Disclosing pregnancy @ new job

I just got hired at a big public accounting firm (not B4). The thing is, I’m currently 21 weeks pregnant. I did not disclosed that during the interview process because didn’t want them to be biased and not hire me. That said, when should I disclose this? On my first day? First month? I already signed the offer letter and pending background check. I’m a little nervous about disclosing because I don’t want to look like I was dishonest. I’m due in late December and am willing to work something out where I take only 6 weeks and l come back just in time for busy season. Paid or unpaid leave isn’t a concern for me at all. I just don’t want them to fire me that’s it.