We got the ex-"Boy Meets World" star -- who played Rachel in the later seasons, and who was Trina's on-screen BFF too -- out in L.A. Tuesday on the heels of TMG's big news ... namely, that she was about to be a mom again with her 4th kid on the way.

Of course, the thing that caught everyone by surprise is the fact Trina's in her 50s ... something that Maitland noticed as well, and which she's somewhat incredulous about.

Check it out ... MW tells us she's absolutely thrilled for Trina -- noting she was there when she got pregnant with her first 3 kids while working on the show -- but in the wake of this latest development ... Maitland says she hopes this isn't a publicity stunt or fake.

In terms of why she'd even think that ... we'll let Maitland explain for herself. Basically, she says she just hasn't seen any proof from Trina that she is, in fact, pregnant at this age.

It's also notable that it's generally difficult for women in their 50s to get preggo -- although not impossible -- and while Maitland hopes this is real ... you can tell she has doubts.

This might seem a little rude or out of pocket -- to some anyway -- but Maitland explains where she's coming from here ... saying Trina should document her pregnancy for a number of reasons ... including to help other women and show that geriatric pregnancies are viable.

Maitland also says that Trina can help stamp out stigmas and shaming that goes on with older women who get pregnant ... so in her eyes, there's a lot of good she can do by chronicling her journey. Whether Trina will actually do that or not, of course, is her decision.