There are sphincters between the stomach and oesophagus that prevent reflux of gastric contents into the oesophagus. When the lower oesophageal sphincter is abnormal in function or structure, symptoms of acid reflux can occur.
Later in pregnancy, as the fetus gradually grows in the uterine cavity, it can cause an increase in intra-abdominal pressure, at which point, it may lead to abnormal function of the lower oesophageal sphincter, which can cause symptoms of acid reflux.