What is the ruling on a woman who delays her obligatory bath because she is waiting to see whether more menstrual blood will come after it stops briefly?

Summary

The woman should check herself. If cotton inserted into the inner part of the vagina shows traces of blood, including brown or yellowish colour, she is still within her menstrual period. If no blood colour appears (naqo'), by default she is considered pure and must bathe promptly and resume prayer and fasting. For a woman with the habit of muwa'adah al-dam, where blood flows, stops and returns with a total of a full day and night, the school has two views: the first (Imam al-Nawawi in al-Majmu') rules her pure, so she must bathe, pray and fast, while the second rules her menstruating, so she need not bathe during the naqo' interval. The fatwa does not explicitly choose between them, but its Nurul example and its reading of the questioner's case apply the second view; if blood does not return by the end of her usual period, she must bathe and make up missed prayers. The answer excludes women with istihadah, first-time menstruation and intermittent bleeding.

Written in our own words from the original fatwa — check the source before acting on it.