My dentist told me that they prefer to do non-urgent stuff in the second trimester.

The reason, he said, is simply that if they do something in early pregnancy and then the pregnancy miscarries then they risk being blamed and sued. And if they do it in late pregnancy and the baby comes early they also risk being blamed and sued. He said there is no clinical reason not to do stuff at those times, the reason is more to do with their insurance costs.

I think that's really sad, actually.

I had a checkup but no work done (cleaning + fluoride treatment only) when I was 5 weeks pregnant (actually the day I got my BFP), and a filling replaced a few weeks ago, at the end of 2nd trimester. No problems with any of that.

Actually, pregnancy can be bad for your teeth and gums, so it's a good idea to check them checked up early on anyway. My OB asked me, on my first visit, whether I'd had a recent dental checkup.