Got heartburn and reflux? You can thank your hormones again for that… Acid reflux is a result of stomach acid entering the oesophagus (food pipe). This occurs during pregnancy due to changes in the hormone progesterone, which relaxes smooth muscles of the body. As a result, the valve between the stomach and the oesophagus is relaxed and consequently allows some gastric acid to enter the oesophagus and cause a burning sensation.