Emergency? If you are soaking through a pad or tampon every hour, feel faint or short of breath, or might be pregnant and are bleeding with pain, call 911 or go to the nearest emergency room. Know the warning signs
Book now

Other causes

The N category, "not otherwise classified," holds causes that are uncommon or still being studied.

By the AUB editorial teamUpdated September 2026PALM-COEIN: AUB-N

Cesarean scar defect

Also called a niche or isthmocele, this is a pouch that can form where a cesarean incision healed. Blood collects there and trickles out after a period ends, causing days of brown spotting. It can also cause pain and may affect a future pregnancy. It is seen on ultrasound, and it can be repaired surgically when symptoms warrant.

Arteriovenous malformation

A rare tangle of blood vessels in the uterus, usually after pregnancy or a uterine procedure. It can cause sudden, very heavy bleeding. It is diagnosed with Doppler ultrasound or other imaging and is often treated by embolization.

Other conditions

Thickening of the uterine muscle and other rare conditions also fall into this category.

Sources

  1. Munro MG, Critchley HOD, Fraser IS; FIGO Menstrual Disorders Committee. The two FIGO systems for normal and abnormal uterine bleeding symptoms and classification of causes of abnormal uterine bleeding in the reproductive years: 2018 revisions. Int J Gynaecol Obstet. 2018;143(3):393–408.

Related