Heartburn is a sensation of burning in the chest caused by stomach acid backing up into the esophagus. The burning is usually in the upper and central part of the chest, just behind the sternum (breast bone). The burning can worsen or can be brought on by lying flat or on the right side. Pregnancy tends to aggravate heartburn. Many people experience heartburn and there are a large number of over-the-counter (OTC) medications and home remedies available to treat heartburn or the symptoms of heartburn.