Heartburn is an irritation of the esophagus that is caused by stomach acid.
With gravity's help, a muscular valve called the lower esophageal sphincter, or LES, keeps stomach acid in the stomach. The LES is located where the esophagus meets the stomach -- below the rib cage and slightly left of center. Normally it opens to allow food into the stomach or to permit belching; then it closes again. But if the LES opens too often or does not close tight enough, stomach acid can reflux, or seep, into the esophagus and cause a burning sensation.
Occasional heartburn isn't dangerous, but chronic heartburn can indicate serious problems and can develop into gastroesophageal reflux disease (GERD). Heartburn is a daily occurrence for about 10% of Americans and up to 50% of pregnant women. It's an occasional nuisance for 30% of the population.
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