Here is some info from the American dental health organisation (presumably it's the same as the UK dental health organisation, but your dentist should be able to give all the info and let you know whether you can have x-rays etc. during pregnancy):
PREGNANCY AND ORAL HEALTH
How does pregnancy affect my oral health?
You may experience some changes in your oral health during pregnancy. The primary change is a surge in hormones, particularly an increase in estrogen and progesterone, which is linked to an increase in the amount of plaque on your teeth.
How does a build-up of plaque affect me?
If the plaque isn?t removed, it can cause gingivitis, usually surfacing in the second trimester. If you already have gingivitis, the condition is likely to worsen during pregnancy. If untreated, gingivitis can lead to periodontal disease, a more serious form of gum disease. Pregnant women are also at risk for developing inflammatory, benign growths that develop when swollen gums become irritated. Normally, the tumors are left alone and will break on their own but if a tumor is very uncomfortable and interferes with chewing, brushing or other oral hygiene procedures, the dentist may decide to remove it.
How can I prevent these problems?
Keep your teeth clean, especially near the gum line. You should brush with fluoride toothpaste at least twice a day, and after each meal when possible. You should also floss thoroughly each day. If tooth brushing causes morning sickness, rinse your mouth with water or with anti-plaque and fluoride mouthwashes. Good nutrition, particularly plenty of vitamin C and B12, help keep the oral cavity healthy and strong. Frequent cleanings from the dentist will help control plaque and prevent gingivitis, which will reduce gum irritation and the likelihood of pregnancy tumors.
When should I see my dentist?
If you?re planning to become pregnant or suspect you?re pregnant, you should see a dentist right away. Otherwise, you should schedule a check-up in your first trimester for a cleaning. Your dentist will assess your oral condition and map out a dental plan for the rest of your pregnancy.
Are there any procedures I should avoid?
Non-emergency procedures can generally be performed when pregnant, but the best time for any dental treatment is during the 4-6 month period. Women with dental emergencies can be treated during any trimester, but your obstetrician should be consulted if anaesthesia or medication is prescribed. Lastly, elective procedures that can be postponed should be delayed until after the birth.