I had endodontic work on a failed root canal when pregnant.
I was nervous about it. More from X-ray perspective. But apparently the level of radiation is so low it's not significant.
I didn't think about the mercury. ( My crown is porcelain but it has some metal under it and I'm not sure what. ) Until afterwards when I needed a filling while breastfeeding. As you may know, the NHS gives free dental during and after pregnancy. But it doesn't cover white fillings.
So I read around and found something from NICE that recommended avoiding during pregnancy and breastfeeding. As a precaution rather than conclusive evidence from studies. I bookmarked the link but it no longer works! But as you know, the NHS doesn't always put it practice those guidelines.
It's not conclusive that getting an amalgam filling in mouth when pregnant or breastfeeding would harm your baby. But they say that the exposure to mercury from fillings is greatest during the placement or removal. If they drill it it could release vapours I think. They say using a dam in mouth is a good precaution.
What tooth is it? If it was a back one I would seriously consider just removing it. So often root canals don't work and later on more work is needed. This happened to me. I personally don't know anyone who had a root canal and says it's never caused them a problem since. Though I'm not saying these people don't exist!
Mine is on a back tooth and I wish now I'd just pulled it. I spent so much money on it (privately) and then it didn't work 100% as there is so often something that only an endodontist can see. It was like a constant minor infection for my body until they fixed it.
I guess if it is a front tooth it might be more straightforward as less roots.