During pregnancy, the placenta produces large amounts of the hormone progesterone, which relaxes the uterus. It has a similar effect on other smooth muscles, such as the intestines and the valve that usually keeps stomach acids out of the esophagus. Relaxation of this valve allows those acids to then make their way back up the digestive tract, resulting in the unpleasant burning sensation known as heartburn. As your baby and uterus get bigger, they push against your stomach, which can compound the problem. If you suffer from heartburn, you may not be able to eliminate it entirely, but there are steps you can take to minimize your discomfort.