During pregnancy the mucous membranes in your mouth swell, and your saliva changes. Your resistance to bacteria also decreases, which can result in gum inflammation and cavities. Increased sugar cravings and eating more frequently (which often helps with nausea) increase your risk of tooth decay further. You should therefore be even more careful about oral hygiene during pregnancy. Use toothpaste with fluoride, rinse with a fluoride mouthwash, and remember to floss. During pregnancy your gums may bleed easily when you brush your teeth. If this happens, try to brush more gently.
Source: - Lärobok för barnmorskor. Faxelid, E. Course literature 2001. - Myles Textbook for Midwives. Fraser, D. och Cooper, M. Churchill Livingstone 2003.