Question:Is there basis for only dunking once in the mikveh? I know the general custom is to dunk at least twice, but I experience a lot of anxiety prior to tvilah and I find that dunking only once calms my anxiety somewhat beforehand.
Yes, the basic halacha is that a single immersion in the mikveh makes a woman tehorah. Ashkenazi women have the custom to dunk twice, but, in extenuating circumstances such as your situation, a single immersion is permissible.
When dunking only once, you should recite the bracha before immersing.
Please write back with any further questions.
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