A hiatal hernia happens when part of the stomach pushes up through the opening in the diaphragm where the esophagus passes through. This opening, called the esophageal hiatus, is normally just 7 to 10 millimeters across. In people with a hiatal hernia, it widens to 16 to 21 millimeters, enough for stomach tissue to slide into the chest cavity. The process is usually gradual, driven by a combination of weakening tissues, increased abdominal pressure, and aging.
The Structure That Holds Everything in Place
Your diaphragm is a dome-shaped muscle that separates your chest from your abdomen. The esophagus passes through a small gap in the diaphragm to connect with the stomach just below. A band of flexible tissue called the phrenoesophageal ligament wraps around this gap and anchors the lower esophagus in position, attaching about 2 to 3 centimeters above where the esophagus meets the stomach. This ligament has a specific job: resist the stomach being pulled upward into the chest while still allowing some movement when you swallow or change position.
A hiatal hernia develops when this system fails. The ligament stretches out, typically along the front and sides of the opening. At the same time, the ring of diaphragm muscle around the hiatus can distort and widen, usually toward the side or downward toward the aorta. Once both the ligament and the muscular ring lose their ability to keep the gap tight, there’s nothing stopping the stomach from migrating upward.
What Causes the Tissue to Weaken
Age is the single biggest factor. Hiatal hernia prevalence rises sharply over the decades: about 2.4% of people in their 50s have one, climbing to 7% in the 60s, 14% in the 70s, and nearly 17% by the 80s and beyond. The connective tissue holding the hiatus together simply deteriorates over time, losing elasticity and strength the same way skin and joints do.
There’s also a genetic component. Research published in the journal Gut identified a gene called COL3A1 as a susceptibility gene for hiatal hernia. This gene controls the production of type III collagen, a protein found in flexible connective tissues throughout the body, including the esophagus and diaphragm. When the balance between type I and type III collagen shifts, the tissues around the esophageal junction become either too stiff or too stretchy, making them more prone to giving way. This genetic link was especially strong in men.
Pressure From Below Pushes the Stomach Up
Anything that chronically raises pressure inside your abdomen can force tissue through the weakened hiatus. The most common culprits are obesity, chronic coughing, repeated heavy lifting, straining during bowel movements, and frequent vomiting. These activities create intense, sustained pressure against the diaphragm from below, gradually widening the opening over months or years.
Obesity deserves special attention. People with morbid obesity (a BMI of 40 or higher) have significantly higher rates of hiatal hernia, and research has found that waist circumference specifically, not just overall weight, correlates with both hiatal hernia and esophageal damage. The excess visceral fat packed around abdominal organs creates constant upward pressure against the diaphragm, even at rest.
Pregnancy can also trigger or worsen a hiatal hernia, though it’s uncommon. The enlarging uterus raises intra-abdominal pressure, and most pregnancy-related diaphragmatic hernias appear after 24 weeks of gestation, when the uterus is large enough to exert significant force. About 21% show up earlier, and another 20% don’t become apparent until labor or postpartum.
The Two Main Types Form Differently
Not all hiatal hernias happen the same way, and understanding the type matters because they behave differently.
Type I (sliding hernia) accounts for the vast majority of cases. The junction where the esophagus meets the stomach slides upward through the hiatus into the chest. It often moves back and forth, slipping up when you lie down or strain, then sliding back into place. This intermittent movement is why it’s called a “sliding” hernia. Most people with acid reflux and a hiatal hernia have this type.
Type II (paraesophageal hernia) is mechanically different. The esophageal junction stays in its normal position below the diaphragm, but the upper portion of the stomach (the fundus) rolls up alongside the esophagus and pushes through the hiatus. This type is less common but more concerning because the herniated stomach can become trapped or have its blood supply compromised.
Type III combines both patterns: the junction and the fundus both migrate upward. Type IV is the most extreme, where organs beyond the stomach, such as the colon or spleen, push through the widened hiatus into the chest cavity.
A Feedback Loop With Acid Reflux
There’s a chicken-and-egg relationship between hiatal hernia and acid reflux. One theory holds that years of acid exposure cause the esophagus to shorten slightly due to scarring and inflammation, gradually pulling the stomach upward. At the same time, once a hiatal hernia forms, it makes the junction between the esophagus and stomach more distensible, meaning acid flows back up more easily. The genetic research on COL3A1 supports this idea: the same tissue weakness that allows the hernia to form also makes the esophageal lining more vulnerable to acid damage. So each condition can worsen the other over time.
Why Some People Get One and Others Don’t
Hiatal hernias result from multiple factors stacking up rather than a single event. Someone with a genetic predisposition toward weaker connective tissue who also carries excess abdominal weight and has a chronic cough is far more likely to develop one than someone with just one of those risk factors. Age compounds everything by weakening the tissues that might otherwise hold up under pressure.
It’s also worth noting that many people have small hiatal hernias and never know it. These are often found incidentally during imaging or endoscopy for other reasons. A hernia only becomes a problem when it’s large enough to disrupt the normal barrier against acid reflux, causes symptoms like heartburn or difficulty swallowing, or in rare cases allows stomach tissue to become trapped above the diaphragm.