During pregnancy, the growing size of the uterus puts pressure on the stomach. Plus, the hormones released during pregnancy relax the sphincter (a ring-like muscle that encircles the openings and passages of the body). This action slows down the digestion process and allows the acid to rise up the sphincter. This causes problems like, indigestion, heartburn, constipation, and gas, which, in turn, causes a burning sensation in the chest.