Answer

It is not brought in Maimonides or in the Shulchan Arukh. The Mishnah Berurah does bring it up (the source is Niddah 17).
One may be uncertain whether this is a spiritual danger or a physical one, like all the things they prohibited because of danger. Here, however, it seems simply that we are dealing with a physical danger, since it is supposed to cause a miscarriage (though with some strain one could interpret the miscarriage as the result of spiritual factors). If so, then it seems to me that there is no need to be concerned about this nowadays, because the Sages probably operated on the basis of the thinking of their time and were mistaken about this, as in many other things.
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Questioner:
In my opinion, we are forced to say that this is a spiritual danger that results in a miscarriage, since it says there that if the fingernails were moved from their place to another place, they no longer cause harm. There is apparently no physical explanation for how a change of place could suddenly make the fingernails stop being harmful… no?
In any case, either way it is difficult. If this is a physical danger, it is hard for me to understand what exactly they were mistaken about, since they presumably saw with their own eyes that this happens. This is not some complicated probabilistic calculation where we could say they erred.
And if it is a spiritual danger, do we have the ability to say that they were mistaken and that nowadays it does not exist, on the basis of our rational thinking? Or are these matters that only the Sages and the great scholars of the generation understand?
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Rabbi:
As for its being moved from place to place—do you have any explanation for the physical danger in fingernails even without that? So if you are prepared to accept a physical danger without an explanation, why would a change of place refute it? That too would just be another physical effect that we do not understand.
In my opinion, there is really no need to say this is a spiritual danger, because it is very hard to assess and measure such facts. Do you know how many people see with their own eyes that “alternative medicine” works? To establish such a thing requires no small amount of statistical skill, and that was not really something the Sages had.
Beyond that, I am also not sure they understood spiritual dangers either. Where would they have gotten information about such dangers? Through prophecy? On second thought, in my opinion they did not really distinguish between spiritual and physical dangers, and in truth that distinction is not so sharp. You can call some danger that you do not understand a “spiritual danger” simply because you do not understand what is really happening there. Once there is an explanation, it will become a physical danger.
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Questioner:
Does the Talmud’s own claim, which it sometimes brings—“The secret of the Lord is with those who fear Him”—not apply in this case?
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Rabbi:
If you are asking me, then no. I do not see where the Talmudic sages got this “spiritual information” from. Sometimes you have an intuition that feels true to you. But here I do not see how such an intuition could exist. It is not only that I do not have such an intuition; I do not even see how it could be possible for others to have one either, except through prophecy. By the time of the Sages, prophecy had already ceased.