J.O.
A standard rule when it comes to lifting things is about 30 lbs, if say what you are lifting is normal for you then continuing to lift it while pregnant is fine. You wouldn't want to do it like an assembly line, but there is no real harm to the baby. Unless you are high risk. The risk is really to you as you get bigger and more awkward. Just thought I'd put that out there...
As others have said, I would tell the MIL at the very least and then wait until mom and baby have basked in the new mommy glow.