Is it permissible for a pregnant woman, who miscarried one of twins while the other remained, to fast, given that she is experiencing intermittent spotting?

If a miscarriage does not show human formation, then the blood is considered irregular bleeding (dam fasad) and does not prohibit prayer or fasting. If human formation is discernible in the miscarriage, then the blood is considered postpartum bleeding (dam nifas), which prohibits prayer and fasting, and requires making up for the missed fasts. The forty-day period is considered nifas with every birth.