Is it true if a pregnant woman steps on nails, her child will die? if so, why? and does this apply stringency apply to both man/woman/under bat/bas mitzvah?
Chazal (Niddah 17) write that for a pregnant woman to step on nails is dangerous for the infant she is carrying. This certainly does not mean that every pregnant woman who steps on nails will miscarry, but only that one should be careful of it. For this reason, we are careful to dispose properly of our nails in places where women might walk (see Orach Chaim 260).
The simple reason is perhaps because of the shock a woman will experience upon stepping on the nails. This, however, does not concur with the added teaching of Chazal whereby the nails only carry their potency in the place that they fell, and not after they have been moved. This suggests that Chazal were concerned for a metaphysical effect of the nails. Kabbalists have written at length on the nature of nails as the “extremeties” of a person, and how this can damage the new life within a pregnant woman.