Although there is wisdom behind all divine laws, it is for the major part hidden to us. In fact, no divine law has been enacted or legislated without a rationale or reason but it remains concealed to us. It is not permissible for women to offer prayers or fast when they are in their menses. This is one of those rules for which there is wisdom but there is no way we can know it. The only thing we know is that the Quran treats menstruating women as those who are suffering from a temporary illness which naturally causes women to be in discomfort. God, the Exalted, terms the illness as “a discomfort” for women.
The Quranic verses consisting of the rules of Hayz only forbids sexual intercourse with women in their menses. The other prohibitions regarding women in their monthly period are deciphered from the Sunnah and traditions. God, the Glorified, says: “And they ask you about menstruation. Say: It is a discomfort; therefore keep aloof from the women during the menstrual discharge and do not go near them until they have become clean; then when they have cleansed themselves, go in to them as Allah has commanded you; surely Allah loves those who turn much (to Him), and He loves those who purify themselves.”
Rules of Hayz in the Traditions
There are special rules mentioned for menstruating women. Abandoning prayers and fasts is among those rules. It has been narrated from Imam Reza (a.s.) that he said: “No sooner a woman sees her monthly blood than she should give up praying and fasting.”
It has been narrated from the Infallibles (a.s.) under chapter “The Necessity of Giving the Qadha of Fast (not Prayers) for a Woman in her Menses or in Lochia” that it is obligatory for a woman to observe the qadha of the fasts which she missed during her monthly period but it is not obligatory for her to offer the qadha of the prayers and she must not do so.
When it comes to the reason why offering the qadha of the prayers missed during monthly period is not necessary, we refer you to a debate between Imam Sadiq (a.s.) and Abu Hanifa in which reference is made to a good point. Imam Sadiq, peace be upon him, says that the reason or philosophy of an Islamic law cannot be reached through analogy (qiyas). Imam Sadiq (a.s.) asked Abu Hanifa, “Which one of these two is more important; the prayer or the fasting?”
“It is the prayer,” answered Abū-Hanīfah.
Imam Sadiq (a.s.) said, “Why is it then that a woman must settle the fasting that she misses due to menstruation while it is not obligatory upon her to settle the prayers that she misses due to the same?”
Imam Sadiq (a.s.) then added, “Woe to you! How do you then depend upon analogy? Fear Allah and never compare the religious laws out of your own opinions.”
It is therefore clearly understood from the narrations that in such cases one cannot speak about divine laws by philosophizing them and creating reasons which emanate from analogy and the likes.
Thus, as per the narrations and consensus of jurisprudents, women should not offer prayers nor should they observe fast during their menstrual period.
Question 1259 (site: 1451) (Women and Deficiency in Intelligence and Consulting Them)
 - See: Philosophy of Islamic Laws
 -Quran, 2: 222.
 - Quran, 2: 222.
 - Amili, Muhammad bin al-Hasan, Wasail al-Shia, vol.2, p. 334, Chapter of Prohibition of Prayer and Fast and the likes for a Menstruating Women, Jami’ al-Ahadi Software.
 - Wasail al-Shi’ah, vol.2, p. 346, بَابُ وُجُوبِ قَضَاءِ الْحَائِضِ وَ النُّفَسَاءِ، الصَّوْمَ دُونَ الصَّلَاةِ إِذَا طَهُرَتْ
 - It has been narrated from Abi Abdillah (a.s.) that he said: إِنَّ السُّنَّةَ لَا تُقَاسُ أَ لَا تَرَى أَنَّ الْمَرْأَةَ تَقْضِي صَوْمَهَا وَ لَا تَقْضِي صَلَاتَهَا The Sunnah cannot be measured/ likened to anything; don’t you see that a woman should observe the qadha of her fast while she cannot offer the qadha of her prayers. See: Wasail al-Shi’ah, vol.2, p. 346.
 - In another saying, Imam Sadiq (a.s.) says about self-formed opinion and analogy: “The religion of God cannot be measured through analogy.” See: Kulayni, Al-Kafi, vol.1, p. 56.
 - Tabarsi, Ehtijaj, Ghaffari, Mazandarani, vol.3, p. 345, Murtazavi Publications, Tehran (date not mentioned).
 - Naraqi, Muhammad Mahdi, Mu’tamad al-Shi’ah fi Ahkaam al-Shari’ah, p. 300, Comprehensive Ahlul-Bait Jurisprudence Software.