External hemorrhoids develop when veins beneath the skin around the anus become swollen and stretched. About 25% of adults worldwide have hemorrhoids at any given time, and the external type forms when pressure inside the abdomen forces blood into the network of veins (called the hemorrhoidal plexus) that sits just outside the anal opening. That pressure can come from many sources, but the result is the same: the veins engorge, the surrounding tissue stretches, and a painful lump forms.
How Pressure Builds in the Anal Veins
The core mechanism behind every external hemorrhoid is increased intra-abdominal pressure. When pressure rises inside your abdomen, it obstructs the normal outflow of blood from the veins around the anus. Blood pools in those veins, causing them to swell. Over time, or even after a single intense episode, that swelling becomes a visible, palpable hemorrhoid.
Think of it like a garden hose with a kink downstream. Blood tries to drain away from the anal area, but elevated pressure in the abdomen acts as that kink, backing everything up. Anything that raises abdominal pressure, even briefly, can trigger this process.
Straining and Constipation
Chronic straining during bowel movements is the single most common trigger. When you bear down on the toilet, you sharply increase pressure in the abdomen and pelvis. Hard stools make this worse because they require more force to pass, and constipation extends the time you spend pushing. Over months or years, this repeated strain degrades the connective tissue that normally anchors the cushions inside the anal canal, allowing veins to bulge outward.
Diarrhea can also contribute. Frequent, urgent bowel movements mean more time on the toilet and repeated bearing down, even if the stool itself is soft. The pattern matters as much as the stool consistency.
Sitting on the Toilet Too Long
Spending extra time on the toilet is a surprisingly specific risk factor, and it’s distinct from prolonged sitting in general. When you sit in a chair or on a couch, the seat supports your pelvic floor. A toilet seat does not. The opening in the center leaves the pelvic floor unsupported, so gravity pulls directly downward on the hemorrhoidal cushions the entire time you’re seated. As that pressure persists, the cushions engorge.
A recent study in PLOS One highlighted smartphone use on the toilet as a measurable risk factor, simply because scrolling through a phone extends the time people sit there. The fix is straightforward: use the toilet for its intended purpose and leave.
Pregnancy and Childbirth
Pregnancy is one of the most well-documented causes of external hemorrhoids. The growing uterus presses on the pelvic veins, raising pressure in the hemorrhoidal plexus for months at a time. Hormonal changes also relax vein walls, making them more prone to swelling. Then, during labor and delivery, the intense pushing creates an acute spike in abdominal pressure that can produce hemorrhoids even in women who didn’t have them during the pregnancy itself.
Heavy Lifting and Intense Exercise
Any activity that involves holding your breath and bearing down (a maneuver called the Valsalva) drives up abdominal pressure in the same way straining on the toilet does. Bodybuilding carries a particularly clear association. One study of athletes found that bodybuilding practice nearly tripled the odds of hemorrhoidal disease compared to other sports, with an odds ratio of 2.88. High-impact aerobics and activities involving repeated perineal pressure, like cycling and horseback riding, are also commonly flagged by clinicians.
This doesn’t mean you need to avoid exercise. It means that breath-holding under heavy load is the specific mechanical problem. Learning to breathe properly during lifts and avoiding maximal straining can reduce the risk considerably.
Low Fiber Intake
Fiber softens stool and adds bulk, which means less straining to pass it. When your diet is low in fiber, stools tend to be hard and compact, setting the stage for the chronic straining that damages anal tissue over time. The current U.S. dietary guidelines recommend 14 grams of fiber per 1,000 calories consumed, which works out to about 28 grams per day on a standard 2,000-calorie diet. Most Americans fall well short of that target.
Good sources include beans, lentils, whole grains, berries, and vegetables like broccoli and Brussels sprouts. Increasing fiber intake gradually (rather than all at once) helps avoid the bloating and gas that can discourage people from sticking with it. Drinking more water alongside added fiber keeps stools soft rather than bulky and dry.
Obesity and Excess Body Weight
Carrying significant excess weight creates a chronic state of elevated abdominal pressure. Research has shown that higher BMI correlates directly with higher intra-abdominal pressure, and that pressure stresses the pelvic floor continuously, especially when standing. Morbidly obese patients in particular show measurably increased pelvic floor pressure in the upright position. Over time, this constant load on the veins around the anus promotes the same engorgement that straining or heavy lifting causes, just at a lower, sustained level.
Aging and Tissue Breakdown
The connective tissue that anchors the anal canal, its muscles, and its lining weakens with age. Studies of perianal tissue across age groups show that the ratio of connective tissue to muscle tissue increases as people get older. That shift means the structural mesh holding everything in place becomes less elastic and less supportive. Veins that were previously held firmly in position start to sag and bulge more easily under pressure. This is a major reason hemorrhoids become more common in middle age and beyond, even in people whose habits haven’t changed.
When an External Hemorrhoid Becomes Thrombosed
Sometimes an external hemorrhoid develops a blood clot inside it, a condition called thrombosis. This typically follows an acute event: a bout of constipation with hard straining, an episode of intense physical effort, or even a period of severe diarrhea. The spike in pressure ruptures the delicate lining inside the swollen vein, which triggers clot formation. The result is sudden, intense pain and a firm, bluish lump near the anus.
Thrombosed hemorrhoids tend to affect younger adults and are associated with hard stools, constipation, and excessive physical effort. The internal anal sphincter can also go into spasm around the swollen tissue, trapping blood below the surface and making the congestion and swelling worse. While the clot itself usually resolves within a few weeks, the pain during the first few days can be severe enough to interfere with sitting, walking, and sleeping.
Multiple Factors Working Together
In practice, external hemorrhoids rarely have a single cause. A person with a low-fiber diet who sits at a desk all day, spends extra time on the toilet reading their phone, and doesn’t exercise much is stacking several risk factors on top of each other. Add age-related tissue weakening or a few extra pounds around the midsection, and the cumulative pressure on those veins becomes enough to produce symptoms. Understanding that hemorrhoids result from this combination of forces, not one isolated habit, makes it easier to see where small changes can interrupt the process.