Is the blood that comes down after a miscarriage at six weeks gestation to be considered menstrual blood, and if not, how does the wife make up for the prayers she missed over a period of three days?

This blood is Istihadah (irregular bleeding) unless it coincides with the regular menstrual period, in which case it is Hayd (menstruation). It is not Nifas (post-natal bleeding) unless the creation of a human is evident in it after more than eighty days. She must make up the prayers she missed, having thought it was Nifas blood. The make-up prayers should be performed immediately, according to her capacity, in a way that does not cause her harm.