Hemorrhoids form when increased pressure on the pelvic veins causes veins in the anal canal to swell and gradually stretch out of shape. Pressure increases can be caused by rushing to complete a bowel movement, persistent diarrhea or constipation, or other factors including being overweight or pregnant.

Persistent pressure also weakens tissues that support the veins in the anal canal. If those tissues become so weak that they can no longer hold the veins in place, the swollen veins and tissues bulge into the anal canal (internal hemorrhoids) or under the skin surrounding the anal opening (external hemorrhoids).

For some people, hemorrhoids may cause a little discomfort for a limited time. Other people have recurrent bouts of discomfort when hemorrhoids flare up. Some people struggle with hemorrhoid pain, discomfort, and itching much of their lives. The degree and duration of discomfort depend on where the hemorrhoids are.

Hemorrhoids frequently develop during pregnancy because of extra pressure on veins (from the enlarged uterus).

During labor, hemorrhoids may start or get worse because of the intense straining and pressure on the anal area while pushing to deliver the baby. For more information, see the topic Pregnancy.

External hemorrhoids

Because external hemorrhoids may not cause any symptoms, you may not be aware that you have hemorrhoids.

When a vein within an external hemorrhoid gets irritated, blood may clot under the skin, forming a hard, bluish lump. This is known as a thrombosed, or clotted, hemorrhoid. Thrombosed hemorrhoids can be very painful.

Internal hemorrhoids

Small internal hemorrhoids may not grow larger if bowel habits or other factors change to lower pressure on the veins in the bowel.

Large internal hemorrhoids may bulge from the anus. After bowel movements, you may have to push them back through the anus. At worst, large internal hemorrhoids stick out all the time.

In rare cases, hemorrhoids may bulge through the anus and swell. Muscles that control the opening and closing of the anus may cut off a hemorrhoid's blood supply (strangulated hemorrhoid). This may cause the hemorrhoid tissues to die. If this happens, you will feel severe rectal pain and may see blood and pus at the anus. You will need urgent surgery to prevent further complications, such as death of the affected tissue and infection.