When required . If the benefits of having the x-ray outweigh the risks to the fetus then an x-ray should be done. X=rays are often required to aid in proper diagnosis under potentially life-threatening situations. In these types of scenarios it is acceptable to take x-rays on a pregnant woman.
When it's needed. Radiation exposure doses from modern digital dental x-ray units are negligible. Using a thyroid shield in addition to an abdomen shield is recommended. If a pregnant woman is having a serious dental problem that requires treatment, there is no contra-indication to getting an x-ray. There are numerous studies, reports, and official guidelines that support this.