Heartburn is a very unpleasant burning sensation that often extends from the bottom of the breastbone to the lower throat. The pregnancy hormone, progesterone, created by the placenta, relaxes the muscles of the uterus. Unfortunately, it also relaxes the valve that separates the esophagus from the stomach, which allows gastric acids to seep back up the pipe. Progesterone also slows down the rhythmic contractions of the stomach, slowing digestion.
As the baby develops, the growing size of the uterus also leads to heartburn by pushing up the stomach acids.
As far as relief goes, many of the remedies associated with morning sickness will help you with heartburn.
First of all, avoid the foods that cause heartburn: carbonated beverages, alcohol (which should be avoided anyway), caffeine, chocolate, tomatoes, vinegar, highly acid foods like citrus, mustard, mint products, processed meats, and spicy, highly seasoned, fried or fatty foods.
Try chewing gum after eating. Chewing stimulates salivary glands, and saliva can help neutralize acid. If you are having trouble at night, try sleeping propped up on several pillows. Elevating your upper half will help keep stomach acids in the stomach, and aid digestion.