I have heard that during niddah a woman must follow the deoritah rules of tzinius but not necessarily the rabbinic ones. If this is the case then why, for example, is a tshirt fine when it goes above the elbows?
Is there a general principle with regards to how a woman should dress during niddah in the prescence of her husband?
The Shulchan Oruch (YD 195, 7) rules that during days of niddah a husband may not see any uncovered area of his wife that she would not normally be permitted to uncover in the presence of strangers. Therefore, just as she would not be permitted to uncover these areas in public, she may not uncover these parts of her body in front of her husband during the unclean days.
However, there are many other aspects of modesty, even in regard to types of clothing, which are not related to revealing skin. These include not wearing tight clothing, brightly coloured clothing etc. I have heard from my Rabbi that these aspects never apply regarding one's husband.
R' Moshe Feinstein writes (YD2:75) that whatever she normally wears inside the house is fine; in his understanding, when Shulchan Aruch says "her covered parts", that doesn't mean "when she's out in public", it means "when she's home."
And the halacha generally stands to reason that the "covered parts" for which her husband is prohibited from looking means only those places covered even at home when no one but her husband is home, or no men are there at all, as even then women are usually somewhat dressed; it doesn't make sense that the intention was "all skin that would be covered when going out in the market, in front of other people", which would require additional modesty.
(Middle of the top paragraph, second column.)
