My best friends husband is a dentist. There shouldn't be any reason to do an xray for a filling that has fallen out....it should simply just be filled. It is perfectly safe to have cavities filled, and attend regular cleanings during your pregnancy. It is actually recommended.
some suggest waiting for treatment other than cleanings till the second tri, some dont. if they xray you have them double shield you (they should without being asked anyway) the level of radiation in todays xrays are so minimal. you will be fine. its better to have your dental work done, then to wait and have infection come about. i read about oral infections having a link to preterm labor or some problems such as that. i had my teeth cleaned in my early first trimester, very important to take care of your teeth and gums during pregnancy.
They will most likely need to take an x-ray since your filling came out, because it's very likey for decay to be there, otherwise a filling really shouldn't fall out. They take the x-ray to see how far down the decay is going. Make sure you get a note from your OB/GYN saying its okay to take radiographs (the dentist will most likely require it before they do any treatment). Other than that, I don't think you will have any problems.
the dentist will fax over to your ob a slip with their permission for dental work. dont assume just because it fell out there is decay, sure maybe, but not always. after 18 years in the dental field they just sometimes become old and come out.
I'm not saying that there will be decay, but being a dentist, 9 out of 10 times, that's what I've noticed.
My dad is a dentist. He would not make an x-ray mandatory! He said he would just fill the tooth and you could come back for a full workup when the baby was delivered.