I stayed on the Metformin for my entire pg.'s with my 3 lc. I had a m/c after stopping Metformin, so I found some good info (back in 2004) that I shared with obgyn and he thought it was wise for me to stay on it throughout my pg with oldest dd. I did the same with the other two.
I have a blood clotting disorder that wasn't diagnosed until after my 4th m/c (and then not treated until Ben and Caitie). So I not only had to take the baby aspirin, but heparin, prednisone, extra folic acid, progesterone shots, etc.
I have "heard" some women (on another board that I frequent) stopped the metformin after week 12 and did okay. However, the ones I personally know ended up with gestational diabetes. I did not.
I never go off the Metformin (pg. or not) now.
Oh...and another benefit, it also helps with milk supply as some with PCOS have a problem keeping up.
I do get bloodwork done every year to make sure there any problems with liver or what-have-you.
After 13 years of infertility, Metformin was a GODSEND!
HTH