Short answer
It is judged by the same rules as any bleeding, whatever caused it. In the Hanafi school, if it lasts at least 3 days and no more than 10, and comes at least 15 days after your last period ended, it is a period. Shorter spotting, or bleeding inside the 15-day gap, is istihadah: you pray and fast, with wudu each prayer time.
Haid is identified by its length and timing, not by its cause, so withdrawal bleeding and bleeding from an IUD or implant are classed by the normal limits: at least 3 days, at most 10, and at least 15 days of purity between two periods. Bleeding outside these limits is istihadah.
The four schools agree on this.
Explanation
Hormonal pills, implants, injections and IUDs often change bleeding: shorter periods, spotting between them, or long stretches of light bleeding. The Hanafi scholars apply the ordinary rules of haid to this blood. The cause does not change the ruling; the length and timing do.
How to classify it:
- 3 to 10 days, after 15 clean days: your period. Do not pray or fast; do ghusl when it stops.
- Under 3 days in total: istihadah. It breaks wudu, but you pray and fast.
- Starting less than 15 days after your last period ended: istihadah, even if it lasts several days.
- Going past 10 days: your usual days are the period and the rest istihadah. See period lasting more than 10 days.
- No bleeding at all on continuous pills: you are pure throughout.
Spotting that stops and starts. Within the 10 days, a clean day between two days of blood counts as part of the period in the Hanafi school. So spotting on day 1, dry on day 2 and spotting again on day 3 can together make a period. This is where a written record of start and end times helps most.
Long, light bleeding from an implant or IUD. Many women bleed lightly for weeks. Count your period by your last regular habit, and treat the rest as istihadah, praying with one wudu per prayer time if it flows constantly. See what is istihadah. Irregular patterns can get complicated, so write down the dates and ask a mufti. Whether these methods are allowed at all is covered in the pill and the IUD.
Ask a mufti about your own case. This answer explains the general ruling. When the details of your situation matter, as they do here, ask a qualified mufti and give them the full facts. Where to ask a scholar.
The evidence
Quran, Surah Al-Baqara 2:222
وَيَسْـَٔلُونَكَ عَنِ ٱلْمَحِيضِ ۖ قُلْ هُوَ أَذًۭى فَٱعْتَزِلُوا۟ ٱلنِّسَآءَ فِى ٱلْمَحِيضِ ۖ وَلَا تَقْرَبُوهُنَّ حَتَّىٰ يَطْهُرْنَ ۖ فَإِذَا تَطَهَّرْنَ فَأْتُوهُنَّ مِنْ حَيْثُ أَمَرَكُمُ ٱللَّهُ ۚ إِنَّ ٱللَّهَ يُحِبُّ ٱلتَّوَّٰبِينَ وَيُحِبُّ ٱلْمُتَطَهِّرِينَ ﴿٢٢٢﴾
And they ask you about menstruation. Say, "It is harm, so keep away from wives during menstruation. And do not approach them until they are pure. And when they have purified themselves, then come to them from where Allah has ordained for you. Indeed, Allah loves those who are constantly repentant and loves those who purify themselves."
Menstruation has its own rulings, which apply to blood that meets its limits.
Sahih al-Bukhari 306
Sahih... إنما ذلك عرق وليس بالحيضة ...
"That is from a vein, not menstruation."
Bleeding that is not menstruation has a different ruling.
Sahih al-Bukhari 325
Sahih... دعي الصلاة قدر الأيام التي كنت تحيضين فيها ...
"Leave the prayer for the number of days you used to menstruate."
A woman with irregular bleeding goes by her habit.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Taharah, chapter on haid. Sets the limits of haid and of purity between two periods, and the rule of the habit.
- al-Durr al-Mukhtar, Kitab al-Taharah, chapter on haid. A clean gap inside the period, with blood on both sides, counts as haid.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.