It is often accompanied by its good friend acid reflux or acid indigestion. This is a condition where the acid from the stomach actually creeps up the esophagus causing a horrible sensation in your throat and mouth. Heartburn is caused by both hormonal and physical changes in the mother's body. During pregnancy, the hormone progesterone is produced in the placenta, which relaxes the smooth muscles of the uterus. Progesterone also relaxes the valve that separates the esophagus from the stomach thus allowing stomach acids to seep back up in to the esophagus, which causes the sensation of heartburn.
Progesterone also slows down the movement of the stomach muscles, making digestion sluggish.
In the later stages of pregnancy, the growing baby crowds the abdominal cavity, slowing down waste elimination from the body and acting as a propeller pushing the stomach acids into the throat. The bad news is that once heartburn begins in pregnancy chances are good that you will continue to have it until delivery. It will probably intensify as your uterus grows. The good news, most women are delivered from most or all of their heartburn problems the as soon as they deliver their baby. Then the pregnancy hormones lessen and the uterus is no longer waging war and winning space in the abdominal cavity.