During pregnancy:

Finally, although orthodontic treatment can be carried out during pregnancy,  there are risks involved. Gingival hyperplasia is likely to be a problem, and the hormonal  variations in pregnancy sometimes can lead to surprising results from otherwise predictable  treatment procedures. Because of bone turnover issues during pregnancy and lactation, an  orthodontist theoretically should be vigilant about loss of alveolar bone and root resorption at  these time-but radiographs to check on the status of bone and tooth roots are not permissible  during pregnancy. Treatment for a potential patient who is already pregnant should be  deferred until the pregnancy is completed. If a patient becomes pregnant during treatment,  the possible problems should be discussed, and it is wise to place her in a holding pattern  during the last trimester, limiting the amount of active tooth movement. Ref: Proffit-4th ed pg no 319