Hi. I am a dentist and suggest that you have your dentist take a look at them to see if treatment is needed now or can be delayed until after pregnancy. It's hard to say withought looking at them, but it's possible that they are just old, in need of replacement, or may have recurrent decay. Fillings which need replacing are prone for recurrent decay.
Anyway, good luck =)
My dentist told me that pregnancy can cause "issues" with your dental health. Something about the hormones and lack of vitamins going to your mouth since they are all going to the baby.....
I haven't had this happen, but more than likely your fillings have just gotten old. I would doubt its pregnancy related. Its fine to go the dentist and have him check it out. Then you guys can decide if you need to have it fixed now or if it can wait. I will tell you from experience not to leave them unless your dentist says its ok. They can abscess, and that is nasty stuff. Get it checked out for sure. Good luck to you .