I had it a bit the other way around; my blood sugars went so crazy immediately with my pregnancy that my primary and MFM both assumed I had had undiagnosed type 2 before. when I lost that pregnancy my blood sugars went back to pre-diabetes stage. I've been taking metformin and my current RE has no issues with that or my A1C.
Apparently insulin resistance can be exacerbated by progesterone levels.
Basically, it just means you need to be more diligent about tracking blood sugars and it is likely that at some point in the pregnancy it will no longer be able to be controlled by oral meds and diet, and you'll need to start insulin injections.