Hemorrhoids develop when the cushions of blood vessels inside and around the anus become swollen, stretched, or displaced. The core mechanism is straightforward: anything that increases pressure in the lower body or weakens the tissue holding those blood vessels in place can trigger them. But the full list of causes is broader than most people realize, ranging from bathroom habits to hormonal changes to the sports you play.
How Hemorrhoids Actually Form
The rectum contains clusters of blood vessels called sinusoids, surrounded by connective tissue that keeps them anchored in place. Unlike veins in your legs, the veins in this area have no valves to prevent blood from pooling. When pressure in the abdomen rises, blood can’t drain efficiently from these vessels during a bowel movement, causing them to swell. Over time, the connective tissue holding them in place stretches and weakens, allowing the swollen cushions to bulge or slip downward into the anal canal.
This is why so many different causes funnel into the same result. Whether the pressure comes from straining, pregnancy, heavy lifting, or simply sitting too long, the endpoint is the same: blood pools in the rectal vessels, the tissue stretches, and hemorrhoids form.
Straining and Bowel Habits
Chronic straining during bowel movements is the most commonly cited cause. Hard stools force you to bear down, which spikes pressure in the rectal veins and gradually degrades the supportive tissue in the anal canal. But the relationship between constipation and hemorrhoids is more complicated than it seems. Research from the Milwaukee VA Medical Center found that diarrhea, not constipation, was more strongly associated with hemorrhoids. A larger national study reinforced this: the conditions most commonly linked to hemorrhoids were all associated with diarrhea, including colitis, malabsorption, and chronic pancreatitis.
This makes sense when you consider that frequent loose stools still involve repeated pressure on the same tissue, plus irritation and inflammation in the anal canal. So both ends of the spectrum, constipation and diarrhea, can contribute through slightly different pathways.
Time Spent on the Toilet
Sitting on the toilet for extended periods is an independent risk factor, separate from straining. A standard toilet seat offers no support to the pelvic floor, so the longer you sit, the more pressure builds in the hemorrhoidal cushions. Over time, those cushions become engorged.
A study published in PLOS One found that people who use smartphones on the toilet have a higher prevalence of hemorrhoids, largely because phone use extends sitting time well beyond what’s necessary. Smartphone users spent considerably more time per visit than non-users, with many exceeding five minutes. Keeping toilet visits under five minutes appears to be a reasonable threshold for reducing risk.
Pregnancy and Hormonal Changes
Pregnancy creates a perfect storm for hemorrhoid development, and it hits from multiple directions at once. The growing uterus puts direct pressure on the pelvic veins, restricting blood flow from the lower body. Blood volume increases significantly during pregnancy, adding to the load on those vessels. On top of that, rising progesterone levels relax the walls of veins throughout the body, making them more prone to swelling.
Pregnancy hormones also slow down the digestive tract, increasing the likelihood of constipation. The combination of mechanical pressure, hormonal vein relaxation, and harder stools explains why hemorrhoids are so common during the second and third trimesters, and especially during labor and delivery, when the pushing phase generates enormous intra-abdominal pressure.
Heavy Lifting and Intense Exercise
Repetitive heavy lifting is a well-established cause. Every time you lift something heavy, especially while holding your breath, pressure inside the abdomen spikes sharply. This obstructs venous outflow from the rectal area, engorging the hemorrhoidal cushions and degrading the elastic and connective fibers of the pelvic floor over time.
A study of athletes found that bodybuilding had a statistically significant association with hemorrhoid disease, even after accounting for age. Cycling and horseback riding also showed positive associations, likely because prolonged pressure on the perineum restricts blood flow in a similar way. High-impact activities like jumping and running can contribute too, though typically less than heavy strength training. The key distinction is between short, intense bursts of abdominal pressure (like a heavy deadlift) and sustained low-level pressure (like a long bike ride). Both matter, but the mechanism differs slightly.
Aging and Genetics
The connective tissue that anchors hemorrhoidal cushions in place naturally weakens with age. As these fibers lose elasticity, the cushions are more likely to slide downward and prolapse, even without extreme straining. This is why hemorrhoids become increasingly common after age 50, even in people without obvious risk factors like constipation or heavy lifting.
Genetics also play a role. Some people inherit weaker connective tissue or vein walls, making them more susceptible regardless of lifestyle. If your parents dealt with hemorrhoids, your baseline risk is higher. This doesn’t guarantee you’ll develop them, but it lowers the threshold for how much pressure or strain it takes.
Obesity and Liver Disease
Carrying excess body weight increases baseline pressure in the abdomen and pelvis, which makes it harder for blood to drain from the rectal veins throughout the day, not just during bowel movements. This persistent, low-grade pressure can gradually cause the same engorgement and tissue breakdown that straining produces.
Liver disease is a less obvious but significant contributor. In people with cirrhosis, blood pressure in the portal vein system rises dramatically. The hemorrhoidal veins are one of the places where the body tries to reroute that backed-up blood. Hemorrhoidal disease shows up in 40 to 44 percent of cirrhotic patients, a rate far higher than in the general population. In these cases, the hemorrhoids can be especially severe because the underlying vascular pressure is so elevated.
Low Fiber Intake
A diet low in fiber makes stools harder and smaller, which means more straining to pass them. Fiber absorbs water in the digestive tract, creating softer, bulkier stools that move through more easily. The current recommendation from the Dietary Guidelines for Americans is 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.
Increasing fiber intake is one of the most effective ways to reduce hemorrhoid symptoms and prevent new ones from forming. Fruits, vegetables, whole grains, and legumes are all reliable sources. If you’re adding fiber to a previously low-fiber diet, increasing gradually over a week or two helps avoid bloating and gas.
How Severity Progresses
Internal hemorrhoids are classified on a four-point scale based on how far they’ve moved from their original position. Grade I hemorrhoids bulge into the anal canal during a bowel movement but don’t protrude outside. Grade II hemorrhoids push out during straining but slide back in on their own. Grade III hemorrhoids protrude and need to be manually pushed back. Grade IV hemorrhoids are permanently outside and can’t be repositioned.
This progression typically happens over months or years of repeated pressure and tissue damage. Catching and addressing the causes early, whether that means changing toilet habits, increasing fiber, or managing chronic diarrhea, can prevent mild hemorrhoids from advancing to stages that are harder to manage without medical procedures.