Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- Most of the jurists consider divorce during menses valid, although it is undesirable (makruh).
2- The divorce during pregnancy is permissible and valid according to all jurists.
Responding to the questions you raised, Dr. Muzammil H. Siddiqi, former President of the Islamic Society of North America, states:
The Prophet (peace and blessings be upon him) said, “The most hateful permissible thing (al-Halal) in the sight of Allah is divorce.” (Abu Dawud and Ibn Majah)
The spouses should avoid divorce as much as possible. If they have difficulties and problems they should try to work out their differences and seek help from their relatives, friends or professional counselors.
However, if the differences are irreconcilable then divorce is permissible, but it should be done in a decent manner. The Shari`ah has given us clear guidelines as to what is the proper way of divorce and what is the improper way of divorce.
According to the Sunnah, the proper time for divorce is when the wife is not in her menses and the husband has not approached her at all during this “clean condition” (tuhr). During menses, sexual intercourse is not permissible.
So the wife was free from menses and the husband in spite of the natural desire and permission to have sexual intercourse did not come close to her. If he divorces her during this time, this means that the differences are really serious.
Muslim jurists, however, have differed among themselves on the validity of divorce during menses. Most of the jurists consider divorce during menses valid, although it is undesirable (makruh). This means that if a husband divorced his wife during her menses, he committed something makruh, but his wife is divorced from him.
The divorce during pregnancy is permissible and valid according to all jurists. The reason, probably, is that it is not forbidden to have sexual intercourse with wife during her pregnancy. During pregnancy, the husband (or baby’s father) normally has a desire to be close to his wife and wants to see his baby as soon as he/she is born. He also knows that he has to take care of all the expenses of `iddah, pregnancy and after that of childcare.
All these things would naturally discourage him from divorce. But if he still divorces her, then it means that they have serious differences. It is reported in a hadith that Ibn `Umar (may Allah be pleased with him) divorced his wife during her menses. When the Prophet (peace and blessings be upon him) knew about it, he told him to cancel his divorce. The Prophet said, “After her menses are over, he may divorce her when she is “clean” (tahir) or “pregnant” (hamil).” (Muslim)
Allah Almighty knows best.