Hormonal changes during pregnancy can affect the body in a number of ways. Progesterone, the hormone your body produces to keep the lining of your uterus thick during your pregnancy, also relaxes the valve between the stomach and the oesophagus, allowing stomach acid to pass through more easily. This can cause uncomfortable feelings such as nausea or a burning sensation.
This is a companion discussion topic for the original article at https://patient.info/news-and-features/why-is-heartburn-get-worse-in-pregnancy