My first trimester...
...has had no noticeable change in breastmilk supply.
However...I was extremely sore and exhausted (found out later we are expecting twins, so perhaps my body was extra run down...my daughter turned 1 yesterday, so she's still rather young as well).
My breasts didn't balloon to gigantic size like they did with my first pregnancy. I didn't notice any size change. And to be honest, the tenderness wasn't that bad either...I think it may have been half caused by my daughter's new teeth since she's learning how to nurse with a mouthful of them!
Now that I'm just over the line into the 2nd trimester, Fiona has pretty much self-weaned for nutrition (though she still nurses to sleep most of the time, and for snacks or comfort). My supply is lower (but that could be from how comparatively little she nurses now), but since she has milk face when she unlatches, I know she's still getting milk.
From what I have heard, almost every woman is different, though. It probably depends greatly on whether you have a constant nurser (like I did at the beginning) or whether your child is partially weaned.