Preventive dental work while pregnant is essential to avoid oral infections such as gum disease, which has been linked to preterm birth.
Dental work while pregnant, such as cavity fillings and crowns, should be treated to reduce the chance of infection. If dental work is done during pregnancy, the second trimester is ideal. Once you reach the third trimester, it may be very difficult to lie on your back for an extended period of time. The safest course of action is to postpone all unnecessary dental work until after the birth.
However, sometimes emergency dental work, such as a root canal or tooth extraction, is necessary. Elective treatments, such as teeth whitening and other cosmetic procedures, should be postponed until after the birth. It is best to avoid this dental work while pregnant and avoid exposing the developing baby to any risks, even if they are minimal.
About medications used during dental work, lidocaine is the most commonly used drug for dental work. Lidocaine (Category B) does cross the placenta after administration. If dental work is needed, the amount of anesthesia administered should be as little as possible, but still enough to make you comfortable. If you are experiencing pain, request additional numbing. When you are comfortable, the amount of stress on you and the baby is reduced. Also, the more comfortable you are, the easier it is for the anesthesia to work. Dental work often requires antibiotics to prevent or treat infections. Antibiotics such as penicillin, amoxicillin, and clindamycin, which are labeled category B for safety in pregnancy, may be prescribed after your procedure.
Routine x-rays, typically taken during annual exams, can usually be postponed until after the birth. X-rays are necessary to perform many dental procedures, especially emergencies. According to the American College of Radiology, no single diagnostic x-ray has a radiation dose significant enough to cause adverse effects in a developing embryo or fetus. According to the ADA and ACOG, having dental X-rays during your pregnancy is considered safe with appropriate shielding.
Some women may elect to avoid dental work during the first trimester knowing this is the most vulnerable time of development. However, there is no evidence suggesting harm to the baby for those electing to visit the dentist during this time frame.
Also, if non-emergency dental work is needed during the third trimester, it is usually postponed until after the birth. This is to avoid the risk of premature labor and prolonged time lying on your back.
Source: http://americanpregnancy.org/pregnancy-health/dental-work-and-pregnancy