Is the blood of miscarriage in the first months, when the fetus has not yet been formed, considered istihadah (irregular vaginal bleeding) or hayd (menstruation) and nifas (postpartum bleeding)? And what is the evidence for that?

The blood resulting from a miscarriage is not considered nifas (postpartum bleeding) unless the fetus has formed, which does not occur before 81 days of pregnancy. This is the prevailing fatwa. If the miscarriage happens before this period, the blood is istihadah (non-menstrual vaginal bleeding), except for what coincides with the usual period of menstruation, in which case it is hayd (menstrual bleeding), because it is neither hayd nor nifas blood. A woman experiencing istihadah should take precautions, pray both obligatory and supererogatory prayers, and perform ablution for each prayer after its time has entered. It is permissible for her husband to have intercourse with her, as long as she does not fear harm.