Hemorrhoids develop when blood vessels in and around the anus swell under sustained or repeated pressure. About one in four adults worldwide has hemorrhoids at any given time, and the rate climbs higher with age. The underlying causes range from everyday habits like straining on the toilet to deeper biological factors like genetics and the gradual weakening of connective tissue over decades.
What Actually Happens Inside the Anal Canal
Everyone has anal cushions, small pads of blood vessels and connective tissue that help with bowel control. Hemorrhoids form when these cushions enlarge, slip downward, or become inflamed. For years, the leading theory blamed sluggish blood flow and backed-up veins, similar to varicose veins in the legs. But more recent vascular research tells a different story.
Ultrasound studies of the superior rectal artery show that people with hemorrhoids have significantly wider arterial branches feeding the area, averaging about 1.9 mm compared to 0.9 mm in people without hemorrhoids. Blood flow velocity is also roughly triple the normal rate. In other words, hemorrhoids appear to involve too much blood flowing in, not too little flowing out. This increased blood supply engorges the cushions, stretching the surrounding tissue until it bulges or prolapses.
The Role of Connective Tissue Breakdown
Blood flow alone doesn’t explain why hemorrhoids become more common with age. The other half of the equation is structural. The anal cushions are anchored in place by a scaffolding of collagen and smooth muscle fibers. Over time, this scaffolding weakens. Tissue samples from hemorrhoid patients consistently show reduced collagen content, altered ratios of collagen types, and lower overall connective tissue density. As this support network loosens, the cushions slide downward and the blood vessels within them dilate more easily.
This degradation is why hemorrhoids are roughly twice as common in adults as in children, whose connective tissue is still intact. It also explains why hemorrhoids tend to worsen progressively. Once the anchoring tissue starts to deteriorate, even moderate pressure can push a cushion further out of position.
Straining and Toilet Habits
The most common trigger is straining during a bowel movement. When you bear down, pressure spikes in the lower rectum, forcing blood into the anal cushions and stretching the vessel walls. Constipation makes this worse because hard, dry stools require more force to pass. Chronic straining, repeated day after day, gradually damages the connective tissue that holds everything in place.
Time spent sitting on the toilet matters too. The Mayo Clinic specifically identifies prolonged toilet sitting as a contributor because the posture relaxes the pelvic floor while gravity pulls on the anal cushions. Scrolling your phone for 15 or 20 minutes on the toilet keeps those tissues under sustained, low-grade pressure that adds up over weeks and months. The fix is straightforward: if nothing is happening after a few minutes, get up and try again later.
Low Fiber and Hard Stools
Fiber is the single most modifiable dietary factor. It absorbs water in the gut, producing softer, bulkier stools that pass with less effort. The recommended daily intake is about 28 grams for a 2,000-calorie diet, but most people fall well short of that. When fiber intake is low, stools become compact and difficult to move, which means more straining, more pressure, and more opportunity for hemorrhoids to develop or flare.
Drinking enough water works alongside fiber. Fiber without adequate fluid can actually make constipation worse, since the fiber needs water to swell and do its job.
Pregnancy and Hormonal Changes
Hemorrhoids are especially common during pregnancy, particularly in the third trimester. Two forces converge. First, the growing uterus presses directly on the large pelvic blood vessels, slowing the return of blood from the lower body and increasing pressure in the rectal veins. Second, hormonal shifts relax the walls of blood vessels throughout the body, making smaller veins in the pelvis and legs more prone to swelling. Add the straining of labor and delivery, and it’s clear why many women develop hemorrhoids for the first time during or shortly after pregnancy.
Excess Weight and Pelvic Pressure
Carrying extra weight, especially around the midsection, creates a chronic version of the same pressure that straining produces. Abdominal fat acts like a constant internal load compressing the pelvic floor and forcing rectal veins to stay partially distended. The effect is present whether you’re walking, sitting, or standing. Over time, this steady pressure can also shift the alignment of the rectum and pelvic floor, making bowel movements mechanically harder and increasing friction on the rectal veins even without deliberate straining.
Heavy Lifting and Exercise
Any activity that spikes abdominal pressure can contribute to hemorrhoids. Heavy weightlifting is a common culprit, especially when form breaks down. Holding your breath while lifting (the Valsalva maneuver) dramatically increases pressure inside the abdominal cavity, which transmits directly to the rectal veins. Squats with heavy loads, crunches, sit-ups, and high-intensity training programs like CrossFit can all aggravate the problem.
This doesn’t mean you need to stop exercising. Lighter weights with proper breathing, exhaling during exertion instead of holding your breath, significantly reduces the pressure spike. The goal is to avoid the combination of heavy resistance and a locked core that turns your abdominal cavity into a pressure chamber.
Prolonged Sitting Beyond the Toilet
Desk jobs and long commutes keep pressure on the pelvic floor for hours at a stretch. While sitting in a chair doesn’t concentrate pressure the same way a toilet seat does, it still reduces blood return from the lower body and keeps the rectal veins under mild but persistent load. People who sit for most of the day without regular movement breaks are at higher risk, and the effect compounds over years.
Genetics and Family History
Some people are simply built with weaker vascular and connective tissue. A 2025 review of 250 studies identified several genes linked to hemorrhoid susceptibility, including genes involved in blood vessel structure, inflammation, and connective tissue integrity. Mutations in these genes can impair the lining of blood vessels, alter the organization of the supportive tissue matrix, and reduce the ability of smooth muscle to maintain vessel tone.
One gene, FOXC2, has been connected to both hemorrhoids and varicose veins through a frameshift mutation that disrupts normal vascular development. Others, like MYH9 and the clotting factor gene F5, have specific single-point mutations associated with higher hemorrhoid risk. If your parents or siblings have dealt with hemorrhoids, your own connective tissue and vascular walls may be inherently more vulnerable to the same pressures that other people tolerate without symptoms.
Age as a Compounding Factor
Aging ties all of these causes together. Connective tissue loses collagen. Blood vessels become less elastic. Decades of bowel movements accumulate wear on the anal cushions. Physical activity often decreases, which can worsen constipation. And conditions that raise abdominal pressure, like weight gain or chronic coughing, become more common. The pooled adult prevalence of about 25% likely underestimates lifetime occurrence, since many people experience hemorrhoids intermittently rather than constantly. Studies that measure whether someone has ever had hemorrhoids push the rate closer to 46%.
None of these causes operates in isolation. A person with a genetic predisposition who also eats a low-fiber diet, sits for long hours, and strains regularly is layering risk factors on top of each other. Conversely, addressing even one or two of the modifiable factors, more fiber, less time on the toilet, better lifting technique, can meaningfully reduce the pressure that drives hemorrhoid formation.