All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
Allah, The most Exalted, said (What means):" They ask thee concerning women’s courses. Say: They are a hurt and a pollution: So keep away from women in their courses, and do not approach them until they are clean. But when they have purified themselves, ye may approach them in any manner, time, or place ordained for you by God. For God loves those who turn to Him constantly and He loves those who keep themselves pure and clean." [Al-Baqara/222].
In principle, a woman shouldn't be hasty in judging her purity from menses (Periods) or confinement(Post-delivery bleeding)-particularly the one who has menses on regular basis-until she sees the white discharge seen by some women or till the blood discharged from her womb ceases.
In Al-Moatt`a, Al-Imam Al-Malik (May Allah have mercy on his soul) narrated (What means):"A'isha, umm al-muminin, said, "Women used to send little boxes to A'isha, umm al-muminin, with a piece of cotton cloth in each one on which was yellowness from menstrual blood, asking her about the prayer. She said to them, 'Do not be hasty until you see a white discharge." By that she meant purity from menses."
Moreover, if a sexual intercourse did take place during menses,by mistake or forgetfully, then the spouses are cleared from the liability, because Allah has pardoned [or been lenient with] for the Prophet and His Ummah their mistakes, their forgetfulness, and that which they have been forced to do under duress. He, The Almighty, said (What means):"Our Lord! Condemn us not if we forget or fall into error." [Al-Baqara/286].
Al-Khateeb Al-Sharbini (May Allah have mercy on his soul) said (What means):"Intentional sexual intercourse with wife during menses is prohibited, and whoever legalizes such an act is considered a disbeliever. However, this excludes doing it by mistake, forgetfully, or being forced to do so under duress. This is because the Prophet (PBUH) said (What means):"Verily Allah has pardoned [or been lenient with] for me my ummah: their mistakes, their forgetfulness, and that which they have been forced to do under duress." A hasan hadeeth related by Al-Bayhaqee and others."[Moghni Al-Mohtajj, 1/110]. And Allah knows best.