As a dental assistant, and also a pregnant one, I have to say that every dentist thinks differently on this topic.
The only worries I would have would be if you had to have more than one or two xrays and ifthe dentist uses local anaesthetic that contains adrenalin also I would stay away from the happy gas, there has been reports and studies on it causing miscarriages (you would have to be working with it to be affected though)
At our practice we dont stop doing anything on pregnant peoples, we do however, use digital xray and our anaesthetic has no adrenalin. Digital xray uses .01 of a seconds exposure so it is actually less radiation than sitting in front of a computer all day.
I myself wouls probably wait for anything other than a check and clean untill though the first tri. Thats just me!! Iam actually in the process of having some porcelain veneers put on and I get the new ones tomorrow morning which I am very excited about!!!! YAY, I have wanted my old ones to be replaced for years but its like any occupation, when you work in it you never have time for any of it yourself (untill a patient cancells)