Final answer:
During pregnancy, heartburn is caused by the relaxation of the lower esophageal sphincter due to pregnancy hormones and the pressure exerted on the stomach by the growing uterus.
Step-by-step explanation:
The gastrointestinal changes during pregnancy that cause heartburn include the relaxation of the lower esophageal sphincter. This relaxation is caused by pregnancy hormones, such as progesterone, which can make the sphincter at the bottom of the esophagus relax more frequently, allowing stomach acid to escape into the esophagus resulting in heartburn or acid reflux. Additionally, the growing uterus puts pressure on the stomach, which can also contribute to the reflux of acid into the esophagus, exacerbating heartburn.