Do you mean you are going for Medicals for visa and you are pregnant? They will ask you if you are pregnant before X-ray. Tell them you are. They are doing X-ray to rule out existing Tuberculosis in Lungs. There are other ways to check that in case you are pregnant.
Please do not go for X-ray In first trimester of pregnancy as it may cause congenital malformations in the baby. If X-ray is must due to unavoidable conditions, they give a protective clothing to be wrapped around your abdomen to protect baby from any exposure.