How can one differentiate between menstrual blood and abnormal bleeding if the bleeding continues after forty days of postnatal bleeding and the ritual purification bath has been completed?

If the blood coincides with the menstrual period after the forty days of postnatal bleeding (nifas) have ended, then it is menstruation (hayd). If it does not coincide with the usual period, then it is irregular bleeding (istihadah). A woman with istihadah should perform ablution (wudu) for each prayer after its time has begun, and she may pray as many obligatory and supererogatory prayers as she wishes with that ablution.