Congrats on the pregnancy, Naomi! I don't take metformin, but I do work in healthcare and part of my job is with high-risk pregnancies (I hate the sound of that, it's mostly people with chronic conditions and are pregnant). I will second what BW has told you: Metformin can be taken safely thoughout pregnancy, although it does not have a product licence for this. It is usually discontinued, although if you've had previous miscarriages after discontinuing the drug then you are usually kept on it until the second trimester at least. I deal with no small number of pregnant PCOS Metformin women: the main reason it's discontinued appears, to me, to be a cost thing as coing off the drug doesn't seem to give any benefit to the mother or foetus. But that's the NHS for you!

Also, metformin doesn't even help every overweight person lose weight: it is used in diabetics to help this, but they don't always lose weight. Metformin is a great drug for type IIs who don't want to be on insulin, but need something other than diet to sort out their hormones. PCOS women usually have some degree of insulin resistance and it's vitally important for a young developing embryo and foetus to have that sorted out - if it's not then there could be developmental problems. This is why most PCOS specialists - not always a obs/gyne, could be an endocrinologist - will leave you on Metformin for the first trimester if coming off it has caused problems before.

In Australia, you pay for all your drugs anyway, right? So you should have no problem with staying on the drug for as long as you need to.