Intercourse with a menstruating woman is not permissible until the bleeding ceases, she enters the state of istihāḍah, and she has performed the full ritual bath; ghusl is therefore indispensable, for Allah the Exalted says: “Do not approach them until they are pure (ṭuhr here means the cessation of menses); when they have purified themselves—that is, bathed—then approach them from where Allah has commanded you” [al-Baqarah 2:222]. This is the position of the majority. See al-Dārimī 249–251 and Nīl al-Marām by Ṣiddīq Ḥasan Khān.

As for the woman experiencing istihāḍah, we know of no explicit sunnah text from the Prophet ﷺ that deals with her in particular. The scholars have differed on whether intercourse with her is allowed; the majority deem it permissible, and that is the correct view. The default ruling for matters is permissibility, and a prohibition would impose hardship on the husband when the istihāḍah is continuous, as occurred with Umm Ḥabībah bint Jaʿsh, mentioned earlier. Most fitting here is what al-Dārimī 207 narrates with a sound chain from Sālim al-Afṭas:

Someone asked Saʿīd ibn Jubayr, “May one have intercourse with a woman in istihāḍah?” He replied, “Prayer is more serious than intercourse.”

A similar statement is reported from Bakr ibn ʿAbd Allāh al-Muzanī, also with a sound chain.

The minimum duration of menstruation is a single discharge: when a woman sees black blood from her private part she must refrain from prayer and fasting… but if she sees only a trace of red blood… she is considered pure. Al-Muḥallā 2/191.

22 – Post-partum bleeding (nifās)

Its maximum period is forty days. Umm Salamah said, “In the time of the Messenger of Allah ﷺ a woman in nifās would remain (away from prayer) for forty days, and we used to apply wars—a yellow plant used as a dye—to our faces because of the freckles (kalaf) that appeared.” D (50) T (254) Mi (229) Maj (223) Qat (82) Mus (175) Ham (4/300, 303, 304, 309)