A hiatal hernia happens when part of your stomach pushes up through a small opening in your diaphragm, the sheet of muscle separating your chest from your abdomen. That opening, called the esophageal hiatus, normally fits snugly around your esophagus. When the surrounding muscle weakens or stretches, the gap widens and allows the upper portion of the stomach to slide upward into the chest cavity. The causes range from years of accumulated pressure on the abdomen to age-related muscle weakening, and sometimes a combination of both.
How the Hernia Actually Forms
Your esophagus passes through the diaphragm via the esophageal hiatus before connecting to your stomach. Bands of muscle called the diaphragmatic crura surround this opening and help keep it tight. A hiatal hernia develops when these muscles weaken, widen, or stretch enough for the stomach to push through.
In about 99% of cases, the result is a “sliding” hernia: the junction where the esophagus meets the stomach moves upward into the chest. This matters because that junction normally sits in the abdomen, where surrounding pressure helps keep it closed and prevents stomach acid from flowing backward. Once it slides into the low-pressure environment of the chest, it loses that support and becomes a less effective barrier. This is why hiatal hernias so often go hand in hand with acid reflux.
The remaining 1% of hiatal hernias are paraesophageal, meaning a portion of the stomach bulges up alongside the esophagus rather than sliding with it. In these cases, the stomach can rotate as more of it pushes through the widened gap. In the rarest form (about 0.1% of cases), other abdominal organs follow the stomach into the chest.
Pressure on the Abdomen Is the Biggest Driver
Anything that repeatedly or chronically increases pressure inside your abdominal cavity can gradually wear on the diaphragm and widen the hiatus. The most common sources of that pressure include:
- Chronic coughing or sneezing, such as from allergies, asthma, or long-term smoking
- Straining during bowel movements, especially from ongoing constipation
- Frequent vomiting
- Heavy lifting or intense exercise, particularly movements that spike abdominal pressure like squatting or deadlifting
- Pregnancy and childbirth, which combine a growing uterus pressing upward with hormonal changes that relax smooth muscle tissue
- Chronic obesity (a BMI over 30), which places constant upward force on the diaphragm
These forces don’t typically cause a hernia overnight. They work over months or years, slowly stretching the hiatus until the stomach can push through. That’s why hiatal hernias are far more common in middle-aged and older adults: decades of wear accumulate.
Age and Muscle Weakening
The diaphragm, like every muscle in the body, loses strength and elasticity with age. The connective tissue surrounding the hiatus becomes less resilient, making it easier for the opening to stretch under pressure. This age-related weakening is one reason hiatal hernias become increasingly common after 50. Many people develop small sliding hernias without ever knowing it, because the hernia produces no symptoms unless it’s large enough to disrupt the anti-reflux barrier.
Obesity and Body Weight
Carrying excess weight, especially around the midsection, creates persistent upward pressure on the diaphragm. Unlike a single episode of heavy lifting, obesity applies that force around the clock. The combination of constant abdominal pressure and gradual muscle weakening makes obesity one of the strongest modifiable risk factors. Losing weight won’t reverse a hernia that’s already formed, but it can reduce the pressure that worsens it and ease related symptoms like reflux.
Smoking
Smoking raises your risk through at least two mechanisms. It triggers chronic coughing, which repeatedly spikes abdominal pressure. It also weakens connective tissue throughout the body by reducing blood flow and impairing the repair processes that keep muscles and ligaments strong. People who smoke are at higher risk for hiatal hernias, and quitting removes one of the ongoing stressors on the diaphragm.
Injury and Birth Defects
Less commonly, a hiatal hernia can result from a direct injury to the diaphragm, such as from trauma or surgery in the area. Some people are also born with an unusually large hiatus, which makes them more vulnerable from the start.
Congenital diaphragmatic hernias, where the diaphragm doesn’t form properly before birth, are a separate and more serious condition. These involve larger defects that allow abdominal organs to crowd the chest cavity during fetal development. In over 80% of those cases, no specific genetic cause is identified. In the remaining 10 to 15%, the defect appears as part of a broader genetic syndrome. These congenital hernias are distinct from the typical hiatal hernias that develop in adulthood, which are overwhelmingly the sliding type caused by gradual wear and pressure.
Why Some People Get Them and Others Don’t
The honest answer is that in many cases, no single cause can be pinpointed. Most hiatal hernias result from a combination of factors: some degree of inherited variation in the size and strength of the hiatus, years of accumulated abdominal pressure from body weight or physical strain, and the natural weakening of muscle tissue with age. Two people with similar lifestyles may have very different outcomes, because the anatomy they started with differs. What is clear is that the modifiable risks, particularly obesity, chronic constipation, smoking, and repeated heavy straining, are worth addressing whether you’re trying to prevent a hernia or manage one you already have.