Aside from the growing baby putting pressure on your organs, an increase in the hormone progesterone can cause your lower oesophageal sphincter (the muscular valve between the stomach and oesophagus) to relax, allowing stomach acids to flow back up into your esophagus.
Sometimes eating little and often, avoiding spicy foods and foods with a high fat intake can help. Avoiding eating late ar night may help too.
Sometimes you may have to visit the doctor for a prescription of antacids to ease the burn.