What is the anatomical mechanism by which pregnancy often causes heart-burn?

During pregnancy, the placenta produces the hormone progesterone, which relaxes the smooth muscles of the uterus. This hormone also relaxes the valve that separates the esophagus from the stomach, allowing gastric acids to seep back up, which causes that unpleasant burning sensation.

Progesterone also slows down the wavelike contractions of your esophagus and intestines, making digestion sluggish. Later in pregnancy, your growing baby crowds your abdominal cavity, pushing the stomach acids back up into the esophagus.

Many women start experiencing heartburn and other gastrointestinal discomforts in the second half of pregnancy. Unfortunately, it usually comes and goes until your baby is born.