Internal hemorrhoids develop when the cushions of tissue lining the upper anal canal become engorged with blood and start to bulge. These cushions are normal anatomy, present in everyone, but a combination of pressure, straining, and gradual tissue weakening causes them to swell and eventually prolapse. Understanding what drives that process can help you reduce your risk or prevent existing hemorrhoids from getting worse.
How Internal Hemorrhoids Form
The inside of the anal canal contains clusters of blood vessels, small arteries, and veins nestled within a layer of connective tissue just beneath the surface lining. These vascular cushions help with continence and are a normal part of the body’s design. Problems start when the connective tissue anchoring those cushions to the muscular wall of the anal canal weakens or stretches. Once the tissue loses its grip, the cushions slide downward, blood pools inside them, and the vessels dilate.
This is why internal hemorrhoids sit above a dividing line in the anal canal called the dentate line. That location matters: the tissue above this line doesn’t have the same pain-sensing nerves as the skin below it, which is why internal hemorrhoids are typically painless, even when they bleed. They progress through four grades, from small swellings that stay in place (grade I) to tissue that protrudes and can no longer be pushed back in (grade IV).
Straining and Prolonged Sitting on the Toilet
The single most well-documented trigger is straining during bowel movements. When you bear down and hold your breath to pass stool, you create a spike of pressure in the veins of the lower rectum. Those veins stretch under that force, and over time, repeated stretching causes them to bulge permanently. A large systematic review of 19 studies found that people with hemorrhoids had roughly twice the prevalence of constipation compared to people without them.
Time spent sitting on the toilet compounds the problem. The seated position on a toilet opening removes support from the pelvic floor, letting gravity pull on the anal cushions while venous pressure stays elevated. Even without active straining, sitting there scrolling your phone for 10 or 15 minutes is enough to keep that pressure sustained. The combination of hard stools, forceful pushing, and extended toilet time is the classic recipe for internal hemorrhoid development.
Chronic Constipation and Irregular Bowel Habits
Constipation doesn’t just cause straining in the moment. It creates a cycle. Hard, dry stool requires more effort to pass, which damages the vascular cushions. That damage triggers swelling, which narrows the canal slightly, which makes the next bowel movement harder. Frequent bowel movements can be just as problematic: people who have loose stools multiple times a day spend more cumulative time bearing down and wiping, both of which irritate the area.
Low-fiber diets are a major contributor here. Without enough fiber to add bulk and moisture to stool, transit through the colon slows, and what arrives at the rectum is compact and difficult to move. Inadequate water intake has a similar effect. Both are modifiable, which makes them among the most actionable causes on this list.
Pregnancy
Hemorrhoids are extremely common during pregnancy, especially in the second and third trimesters, and several factors converge to make them nearly inevitable for many women.
- Blood volume increase: Your body circulates significantly more blood during pregnancy, which forces veins throughout the body to handle a heavier load. Veins in the pelvic region, already under gravitational pressure, are especially vulnerable to swelling.
- Uterine weight: As the uterus grows, it presses directly on the veins that drain the rectum and anus. That compression makes it harder for blood to flow freely back toward the heart, so it pools in the hemorrhoidal vessels instead.
- Hormonal changes: Rising progesterone levels relax the walls of veins, making them more likely to distend. Progesterone also slows digestion, which leads to constipation, adding straining to the mix.
Many pregnancy-related hemorrhoids improve after delivery once the uterine pressure is gone and hormone levels normalize, though they don’t always resolve completely.
Aging and Tissue Weakening
The connective tissue that anchors the anal cushions in place naturally loosens with age. This is a gradual, structural change: the fibers that tether the lining of the anal canal to the muscle beneath it become lax, allowing the cushions to slide downward under even normal pressure. It’s the same general process that causes skin to sag or joints to loosen elsewhere in the body.
This is why hemorrhoids become increasingly common after age 30 and peak between ages 45 and 65. Someone who tolerated years of mild straining without symptoms may find that the same habits now produce bleeding or prolapse, simply because the supporting tissue is no longer as resilient.
Heavy Lifting and Physical Strain
Any activity that forces you to hold your breath and bear down raises pressure inside your abdomen, and that pressure transmits directly to the rectal veins. Weightlifting is the most obvious example. The mechanics are nearly identical to straining on the toilet: you brace your core, trap air in your lungs, and push. That drives force downward onto the pelvic floor.
This doesn’t mean exercise causes hemorrhoids in every case, but heavy lifting with poor breathing technique over months or years is a recognized contributor. Jobs that involve frequent heavy lifting carry a similar risk. Proper breathing during exertion, exhaling as you lift rather than holding your breath, reduces the pressure spike considerably.
Obesity and Sedentary Lifestyle
Excess body weight, particularly around the abdomen, creates sustained pressure on the pelvic veins in much the same way a pregnant uterus does. The effect is constant rather than intermittent, which means the venous cushions are under load throughout the day. Prolonged sitting at a desk or in a vehicle adds to this by reducing blood flow from the pelvic region and keeping mild pressure on the anal area for hours at a stretch.
Physical inactivity also slows bowel motility, making constipation more likely, which circles back to straining. Regular movement, even walking, promotes healthy bowel function and improves venous return from the lower body.
What About Liver Disease?
It’s a common assumption that liver disease and the elevated venous pressure it causes would lead to hemorrhoids, since the veins draining the rectum connect to the portal venous system. Research, however, doesn’t support this link clearly. A study measuring venous pressure in patients with cirrhosis found no significant difference in pressure levels between those with and without hemorrhoids. The swollen veins that sometimes appear in the rectal area of cirrhosis patients (anorectal varices) are a separate entity from true hemorrhoids, though they can look similar. Liver disease may worsen existing hemorrhoids, but it doesn’t appear to be a primary cause of them.
Putting the Causes Together
Internal hemorrhoids rarely result from a single factor. They typically develop when several causes overlap: aging tissue plus a low-fiber diet, or pregnancy plus chronic constipation, or a desk job plus heavy gym sessions. The common thread across nearly every cause is sustained or repeated pressure on the vascular cushions of the anal canal, combined with weakened connective tissue that can no longer hold those cushions in place. Targeting the modifiable factors, particularly fiber intake, toilet habits, hydration, and physical activity, addresses the most common drivers regardless of which specific combination applies to you.