Halacha requires a woman to thoroughly clean her teeth before immersing in the Mikveh. She obviously does not open her mouth during immersion, but, nevertheless, the inside of her mouth must be in a state that allows for unobstructed contact with the water in the Mikveh, even though this contact itself is not required. Thus, just as she must remove all foreign particles from her body so that they do not come in between her body and the water, so must she ensure to remove all foreign particles from the inside of her mouth.
For this reason, it is customary for women to refrain from eating meat during the day before their immersion in the Mikveh. Meat particles often become wedged in between the teeth, and would thus constitute a "Hasisa" (obstruction) as far as immersion is concerned. Therefore, as the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) mentions, there is a custom for a woman to refrain from meat on the day prior to her immersion.
If a woman immersed and upon arriving home she noticed a piece of meat wedged in between two teeth, must she remove the particle and then return to the Mikveh to repeat the immersion? Meaning, does the presence of a meat particle disqualify an immersion even after the fact, or does this Halacha apply only on the level of "Le'chatehila" optimally, but not as an indispensable prerequisite?
Hacham Ovadia Yosef, in his work Halichot Olam, rules that the woman does not need to repeat the immersion in such a case. He notes that the requirement to remove foreign particles from the inside of one's mouth applies only on the level of "Mi'de'rabbanan" by force of Rabbinic enactment, and not on the level of Torah law. As such, there is greater room for leniency in this regard. Moreover, the Meiri (Talmudic commentator, France, 1249-1315) maintained that the presence of a foreign particle disqualifies immersion only in the case of a piece of bone, or similarly solid substance. A piece of meat, however, does not render one's immersion disqualified. Finally, Hacham Ovadia adds, it is possible that the piece of meat was not actually wedged firmly in between the teeth during the time of the immersion. Therefore, due to these factors, in such a case the woman is considered "Tehora" (pure) and does not need to repeat the immersion.
Of course, this ruling applies only after the fact; optimally, however, a woman is required to thoroughly clean her teeth and ensure to remove all foreign particles as part of her preparation for immersion in the Mikveh.
(For more on this topic, see "Eating Meat on the Day of Immersion in a Mikveh; Immersing with Braces, a Retainer or Temporary Fillings," dated January 7, 2007)
Summary: A woman must thoroughly clean her teeth and remove all foreign particles from the inside of her mouth before immersing in the Mikveh, and on the day of the immersion she should refrain from eating meat. If a particle of meat was discovered in between her teeth after she immersed, the immersion is nevertheless valid.
