Before we go deeper,

Before pregnancy
It is important to care for your teeth and gums before pregnancy. There is a proven link between gum (periodontal) disease in pregnant women and premature birth with low birth weight, which can put your baby at risk of a number of health conditions. Research shows up to 18 out of every 100 premature births may be triggered by periodontal disease.

Is Having Dental Work During Pregnancy Safe?
Pregnancy and dental work questions are common for expecting moms. Preventive dental cleanings and annual exams during pregnancy are not only safe, but are recommended. The rise in hormone levels during pregnancy causes the gums to swell, bleed, and trap food causing increased irritation to your gums. Preventive dental work while pregnant is essential to avoid oral infections such as gum disease, which has been linked to preterm birth.
What about other regular dental work during pregnancy?
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. 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.
What about x-rays used in dental work during pregnancy?
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.

Oral Health Conditions During Pregnancy

During pregnancy, several oral health conditions are more common:
Gingivitis may result from hormonal changes that exaggerate the response to bacteria in the gum tissue
Dental caries may occur due to changes in diet such as increased snacking due to cravings, increased acidity in the mouth due to vomiting, dry mouth or poor oral hygiene stemming from nausea and vomiting.
Pyogenic granuloma (aka. Granuloma gravidarum) is a round growth, usually connected to the gingivae by a thin cord of tissue, that may develop due to hormonal changes.
Erosion stemming from vomiting as a result of morning sickness may be detected.
Dry Mouth Many pregnant women complain of dry mouth. You can combat dry mouth by drinking plenty of water and by using sugarless hard candies or gum to stimulate saliva secretion and keep your mouth moist. These should contain xylitol, which reduces the harmful bacteria that cause cavities.
Excessive Saliva Less commonly, pregnant women feel they have too much saliva in their mouths. This condition occurs very early in a pregnancy. It disappears by the end of the first trimester. It may occur along with nausea.
Due to the increased risk of gingivitis and caries, the importance of good daily oral hygiene should be emphasized to these patients. Brushing twice a day with a soft-bristled brush for two minutes, using a fluoride-containing toothpaste, and cleaning between the teeth once a day should be encouraged.

MYTHS and FACTS
I've heard that pregnant women lose a tooth for every child. Is that true?
No. This is a myth. People used to think that a developing fetus who did not get enough calcium would absorb it from the mother's teeth. This is not the case. If you practice good brushing and flossing habits, you are no more likely to get cavities or lose teeth during your pregnancy than at any other time.
Your oral health doesn’t affect your baby.
Fact: If you have moderate to severe gum disease, you may be at higher risk for delivering a pre-term, low-birth weight baby.
Pregnant women should avoid dental work.
Fact: Everyone — and especially pregnant women — should visit the dentist. If you’re pregnant, you face a higher risk for gum disease, so make sure to visit your dentist for regular cleanings, exams and any other treatment needed. Skimping on dental care could affect your pregnancy, as well as your dental health. Untreated gum disease may be linked to pre-term and low-weight birth.
What about anesthesia?
Some studies have found a relationship between anesthesia in the first trimester and early miscarriage. If you need treatment requiring anesthesia, your dentist may recommending postponing the procedure until the second trimester.
Pregnancy leaches calcium from your teeth.
Fact: The fetus does not take calcium from its mother’s teeth. This myth likely originated because pregnant women face a higher risk of tooth decay. Pregnancy is a critical time to consume calcium –the essential nutrient provides helps your growing fetus develop properly and lowers your own risk of osteoporosis (bone loss) later in life.
Never get a dental x-ray while pregnant.
Fact: Dental x-rays are now considered safe during pregnancy by the American Dental Association. X-rays can be essential in detecting serious problems, such as hidden decay, bone loss and inflamed tooth pulp. No research has found a link between dental x-rays and birth defects, although a 2004 study did find an increase in low birth weight among women who had dental x-rays while pregnant. If you have any concerns, talk to your dentist, who can help evaluate your case and decide whether x-rays can be postponed.
Brush your teeth immediately after vomiting.
Fact: When vomiting occurs, gastric acid from the stomach enters the mouth. Teeth are not designed to withstand gastric acids, which can erode the enamel on your teeth. This can lead to sensitivity, cavities, and fractures. Brushing immediately after vomiting can further erode vulnerable enamel. Instead, try to rinse with plain water or water with baking soda after vomiting. The baking soda helps to neutralize the acid in your mouth. Then, after 30 minutes, brush with toothpaste containing fluoride.