Preventative dental exams are safe and encouraged during pregnancy. Preventative x-rays should be left until after delivery if possible. In the event that more extensive work is needed that requires an x-ray, studies show one x-ray will not expose a fetus to a harmful level of radiation.
Lidocaine is the numbing agent most frequently used during dental procedures and it does cross the placenta, so should you need dental work done to avoid infection or damage, the smallest dosage of lidocaine possible, while still keeping you comfortable, should be administered.
Antibiotics prescribed to fight infection are considered safe during pregnancy.