It is not permissible for a woman who is bleeding following childbirth to fast, and her fast is not valid. She has to make up the days that she missed because of nifaas (post-partum bleeding).
Nifaas is the blood passed by a woman because of childbirth.
But if a woman miscarries (or has an abortion), the blood that she passes is not considered to be nifaas unless the foetus had human features.
Human features do not appear in the foetus before eighty days of pregnancy, because the Prophet (peace and blessings of Allaah be upon him) said: “The creation of any one of you is put together in his mother’s womb for forty days, then he becomes a ‘alaqah (a piece of thick coagulated blood) for a similar period, then he becomes like a chewed piece of flesh for a similar period, then Allaah sends an angel who is enjoined to write down four things, and it is said to him: ‘Write down his deeds, his provision, his lifespan and whether he is doomed for Hell or destined for Paradise. Then the soul is breathed into him.” Narrated by al-Bukhaari, 3208.
This hadeeth indicates that a human being goes through a number of stages during pregnancy.
For forty days he is a nutfah (mixed drops of male and female sexual discharge, for the next forty days he is a ‘alaqah (a piece of thick coagulated blood), then for forty days he is a mudghah (a lump of chewed flesh), then the soul is breathed into him after one hundred and twenty days have passed.
Human features begin to appear at the mudghah stage, not before that, because Allaah says (interpretation of the meaning):
“O mankind! If you are in doubt about the Resurrection, then verily, We have created you (i.e. Adam) from dust, then from a Nutfah (mixed drops of male and female sexual discharge, i.e. the offspring of Adam), then from aclot (a piece of thick coagulated blood) then from a little lump of flesh — some formed and some unformed (as in the case of miscarriage) — that We may make (it) clear to you (i.e. to show you Our Power and Ability to do what We will)”
Allaah described the mudghah as being formed and unformed. What is meant by formed is that there appear on the embryo traces of how the body will look, such as the head, limbs, etc.
Based on this, if this woman had an abortion, if that happened before eighty days of pregnancy, then the blood she passes is not the blood of nifaas, rather it is istihaadah (non-menstrual bleeding) which does not stop her from praying and fasting, but she has to do wudoo’ for each prayer.
If the abortion took place after the soul was breathed into the foetus – i.e., after one hundred and twenty days of pregnancy – then the blood is definitely nifaas.
If the abortion took place between eighty and one hundred and twenty days, then the embryo should be examined. If it has human features then the blood is nifaas, and if it does not then the blood is istihaadah.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said in his essay al-Dima’ al-Tabee’iyyah li’l-Nisa’ (The Natural Blood of Women), p. 40:
Nifaas is not established unless the embryo expelled has human features. If she expels a small embryo that has no human features then her bleeding is not nifaas, rather it is bleeding from a vein, so it comes under the same ruling as istihaadah. The earliest at which human features may appear is eighty days from the beginning of pregnancy, but it usually happens at ninety days.
The woman in nifaas has to stop praying and fasting until she becomes pure. When the bleeding stops and she becomes pure, she should do ghusl and pray and fast. If the bleeding lasts for more than forty days, if the additional bleeding coincides with her usual menstrual cycle then it is hayd (menstruation); if it does not coincide with her usual cycle then it is istihaadah, so she should do ghusl and pray and fast, and do everything that women who are taahir (pure) do.
And Allaah knows best.