Nisaa
Yes. If your hayd or nifas ends, meaning the bleeding has actually stopped, before Fajr, that day's fast is valid even if you delay ghusl until after Fajr comes in. What decides the fast is whether the bleeding itself is still present, not whether the bath that follows it has been done yet.
The root of this ruling is the hadith of 'Aishah and Umm Salamah (Radi'Allahu Anhuma), who both reported that the Prophet (Salla Allahu Alayhi Wa Sallam) would enter the morning in janabah from intercourse with his wives, not from a dream, during Ramadan, and would then fast that day (Narrated by al-Bukhari and Muslim). Scholars apply the same reasoning to a woman whose hayd or nifas ends before dawn: her fast needs only to begin free of the bleeding itself, exactly as a state of janabah needs only its cause to have happened before Fajr. Janabah does not stop a fast even when it carries past Fajr, for the same reason.
This is consistent with why hayd stops a fast in the first place: it is the bleeding, not impurity in general, that the Prophet (Salla Allahu Alayhi Wa Sallam) pointed to when he asked the Companions, "Is it not the case that when she menstruates she does not pray or fast?" (Narrated by Muslim). Once the bleeding itself has stopped, that reason is gone, and ghusl becomes a separate matter tied to the next prayer, not to the fast.
Two things this does not change. First, you need to actually be certain the bleeding has ended: the two recognised signs of tuhr settle that, not a guess or a pause you are hoping will hold. Second, ghusl still cannot wait indefinitely. It has to be done in time for Fajr prayer, whose window Allah has fixed: "Indeed, the prayer has been decreed upon the believers a decree of specified times" (An-Nisa 4:103). The fast survives a delayed ghusl; Fajr prayer does not.
This is the mirror image of the other Fajr question. If hayd or nifas is still present when Fajr comes in and only ends later in the day, that day's fast does not count at all and must be made up afterward, whatever the schools say about abstaining for the rest of that day. The difference between the two questions comes down entirely to which side of Fajr the bleeding actually stopped on.
The same holds for nifas: what matters is that the bleeding itself has ended before Fajr, not the timing of the bath afterward. A moment that easy to lose track of on a broken night's sleep is exactly what is worth timestamping rather than estimating after the fact; a period tracker built for Muslim women that logs the minute bleeding actually stopped is what turns "was it before or after Fajr" into a fact you can check, not a guess made hours later.