A hiatal hernia occurs when the upper part of your stomach pushes up through your diaphragm and into your chest region. The diaphragm is a large muscle that lies between your abdomen and chest. You use this muscle to help you breathe. Normally, your stomach is below the diaphragm, but in people with a hiatal hernia, a portion of the stomach pushes up through the muscle. The opening it moves through is called a hiatus. This condition mostly occurs in people who are over 50 years old.