Many women develop GERD, or have their symptoms become worse, during pregnancy. Up to half of all pregnant women experience heartburn at some point during their pregnancy, and up to 1 out of 4 suffer from heartburn every day.
Some doctors believe that this happens because the growing baby puts pressure on the stomach, and pushes it up against the diaphragm. Such an increase in pressure on the stomach can cause the lower esophageal sphincter to open, even when it really shouldn't, allowing acid reflux to happen.
Also, normal hormonal changes that happen during pregnancy can weaken the sphincter, making it even MORE likely to open up when it shouldn't. Both the increased pressure on the stomach, and a weaker sphincter can make it easier for acid reflux to happen during pregnancy.