Pregnancy and heartburn all too often seem to go hand in hand. One of the causes of this is the raised hormonal levels that are due to pregnancy. This leads to softening of the ligaments which normally keep the lower esophageal sphincter tightly closed. Therefore, stomach acids are largely unhindered in travelling up to the throat and esophagus.
I had never experienced heartburn until pregnant with twins, it was really painful. I found eating small amounts frequently and avoiding spicy food helpful, but I did take some Tums (good source of calcium) as well.