All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
A woman in menses who has to make Ghusl (have a ritual bath to remove major impurity) after sexual intercourse is not obliged to make Ghusl until the end of her menses. This is the opinion of Imaam Ahmad and others, because making Ghusl will not allow her to perform the prayer, fast, or touch or read the Qur'an and so forth. However, some other scholars are of the view that she has to make Ghusl before becoming pure from menses. Nonetheless, the correct opinion is the first opinion that she is not obliged to make Ghusl.
If she makes Ghusl while in menses, her Ghusl is correct and the impurity from sexual defilement is removed. As regards the menses, its ruling remains until the blood stops coming out; because being pure from sexual defilement does not purify her for prayer as long as she is still in menses.
If she does not make Ghusl to remove sexual impurity till the end of her menses and then intends at the same time to make Ghusl to remove sexual impurity and become pure from menses, then this is sufficient for her, and this is the view of the majority of scholars among whom are the four Imaams.
However, if she only intends by having a bath to remove one of the two conditions, then that is sufficient for what she had intended only, as the Prophet said: "Every person will be rewarded for what he had intended only." [Al-Bukhari] This is the opinion of Imaams Ahmad and Maalik. Some other scholars are of the view that one bath is sufficient for her purification from both conditions because she becomes pure with the same method of purification, so one condition is dropped by doing [purifying for] the other. This is the view of Abu Haneefah and Ash-Shaafi'ee. However, to be on the safe side, it is better for her to intend to remove both conditions with one ritual bath.
Allaah knows best.