Why do pregnant women get acid reflux?

Progesterone: The hormone Progesterone starts to raise rapidly very early in pregnancy. It causes a slowing down of the intestinal tract causing pregnant women to experience heart burn and constipation. Later on as the uterus grows, it exerts pressure on the stomach also causing heart burn.

Hormonal changes: Something called the lower esophageal sphincter (at the lower end of the esophagus, just above the stomach) is looser during pregnancy because of the effect of hormones. This looser valve allows the stomach acid to 'back up' , or, 'reflux' more easily with laying down flat, laying down on the right side, and as the baby gets bigger and pushes up under the stomach.

Hormonal changes: As per previous 2 answers. The answer lies in the balance, rather the loss of balance, of your body's hormone. Essentially, the lower valve of your esophagus loosens allowing reflux, and also the rate at which you digest food may decrease thus increasing time in your stomach. And to not forget, as the baby grows, the pressure on the stomach is enough to overcome the valve pressure.