If one is pregnant may they go to a manicurist’s shop, since there are nails all over the floor?
The Braita (Nidda 17a) teaches that someone who throws nails down on the floor in a place that is frequented by women is acting wickedly because this may (in a mystical way) precipitate miscarriages. The Braita is quoted in the Mishna Berura 160 6 but is not referred to by the Shulchan Aruch or the R'ma. Rav Moshe Feinstein (Iggrot Moshe Yoreh Deah III 20, the teshuva deals with the prohibition against leaving eggs without their shells overnight) raises the possibility that some of the warnings listed in Nidda 17a were not mentioned by the Shulchan Aruch or the R'ma because they are not Halacha. Even if the Braita were to be seen as binding Halacha, the prohibition mentioned in the Braita and quoted in the Mishna Berura refers only to the person who disposes of his nails incorrectly; there is no parallel injunction that prohibits women from going to a place where people are not careful about nail disposal. This could be because the chance of miscarriage is remote; it is forbidden to be a "Mazik"- to endanger others even remotely for no reason but the same reason does not limit the movement of those who may be endangered. In practice- the woman should ask the manicurist to sweep the floor (according to some opinions, nails that are moved from where they fell present no danger even if they remain on the floor) before her nails are cut, but even if he doesn't comply she is not prohibited from entering. She should insist that her own nails are disposed of properly or at least swept to the side until that time when they are disposed of.