Usually, the esophagus has a valve close to the underside that forestalls abdomen contents from washing again up into the throat and mouth.
throughout pregnancy, elevated ranges of hormones may trigger this valve to not work in addition to it always does, or to open on the incorrect instances which permits abdomen acid to rise again into the esophagus and mouth.
Also, the creating embryo places inside strain on the mother's organs (including the abdomen) which can put some upward strain on the abdomen contents.