How/when to tell them I’m pregnant?

I’ve been job searching for months, and I’m finally in the final round of interviews for three different nonprofit organizations. I’m really hoping that one of these comes through! The thing is, I am about six months pregnant. I’m not sharing this information during interviews, which are all conducted via Zoom. I’m not planning to tell them until after paperwork gets signed.

Because I work for nonprofits, I feel bad that I’m going to blindside one of these well-meaning managers with the news that I’m going on maternity leave soon. Some of these are small organizations and teams. What’s the best way for me to tell them, once I do get the job? During the first manager meeting on my first day? Or via email before that? Or some other way? I’m feeling nervous and guilty about it.