Increased hormone levels of progesterone in pregnancy relax all the smooth muscles in your body. Between your stomach and your food pipe to your throat (oesophagus) you have a sphincter which is made of smooth muscle.
As this relaxes in pregnancy it is common to get heartburn – the acid in your stomach rides back up your food pipe, causing an unpleasant burning sensation. This is made worse by the pressure of your baby on your stomach at the end of pregnancy.