I once "accused" (nice word choice, Amyjay!) my T of being pregnant because I could not figure out why she was getting married to her partner so suddenly. She tried to deflect and explore the meaning of my question (one of those "that's quite a personal question, and I'm wondering why you are asking..." things), but eventually she caved and told me that she wasn't pregnant. It is probably relevant to the story that I was in the middle of trying to have a baby at the time.

She's a little beyond childbearing years now, but I have noticed that she has put on a bit of weight during the pandemic. Stress eating and not being able to do one's usual activities can do that kind of thing. Could it be that? Or are you pretty sure she's pregnant?

I admit I would probably feel uncomfortable just asking if it were because of things I noticed about her body. If that is the case, maybe you could ask if she has any long-term leaves planned in the next year or so? That is very relevant to your therapy, and if she says no, it seems quite likely that she isn't pregnant.