Question: Is it sinful to have sexual intercourse with a menstruating woman or for a menstruating lady to hide the fact that she is menstruating from her husband?
ANSWER
Yes, both are sinful acts. When the husband asks his wife whether she is menstruating or not, she incurs a grave sin if she does not tell the truth. As a matter of fact, our Master the Prophet declared, "A woman who conceals the beginning and the termination of her menses from her husband is accursed" (Jawhara).

This means to say that it is a sin to hide her being menstruating from her husband or to deceive him into believing that her menses is still continuing when in reality it has stopped, for Allahu ta'ala has forbidden men to have sex with their wives when they are menstruating. The Qur'an al-karim says (what means):
(Do not approach women during their menstrual periods until they become pure.) [Al-Baqara 222]

It is declared in a hadith-i sharif:
(What is halal when a woman is menstruating is to touch her only over her clothes. What is best is to refrain from this as well.) [Tabarani]

Question: Is it allowed for a husband to touch and benefit sexually from his menstruating wife's bare skin that is between her knees and navel?
ANSWER
There are different rulings regarding a man's getting sexual benefit from his wife's area that is between her knees and navel. What is the safe side is not to touch her bare skin.

According to Imam-i A'zam, the husband can benefit from his menstruating wife on condition that the area between her navel and knees is covered with a cloth. According to Imam-i Muhammad, however, he can benefit sexually from his wife's bare skin that is between her knees and navel with the exception of sexual intercourse with her. (Radd-ul-mukhtar)

When a woman is menstruating, her husband is allowed to have sexual contact with her if the area that is between her knees and navel is covered with a cloth. According to Imam-i Muhammad, however, even if the area between her navel and knees is naked, he can benefit sexually from her with the exception of having sexual intercourse with her. (Mabsut)

The ruling of the Hanbali Madhhab is the same as the ijtihad of Imam-i Muhammad. The husband can act on the ijtihad of Imam-i Muhammad in case of necessity. Two hadith-i sharifs say (what means):

(You can benefit from your menstruating women in all ways aside from sexual intercourse.) [Muslim, Tirmidhi, Nasai, Abu Dawud]

(You can benefit from your menstruating wives after covering their vulvae.) [Ibn Majah, Abu Dawud]