Heartburn may be caused by changes in your hormone levels. One of the pregnancy hormones, called progesterone, can relax the muscle that usually holds your oesophagus closed where it meets your stomach. This allows food and acid from your stomach to go back up your oesophagus. Heartburn becomes more common as your pregnancy progresses. This can happen when your uterus (womb) pushes up against your stomach as your baby grows. This also pushes the contents of your stomach up into your oesophagus. You’re more likely to get heartburn during pregnancy if you’ve had a baby before or if you get heartburn when you’re not pregnant.