Studies regarding the incidence rates during pregnancy find varying rates, ranging from one-quarter of all pregnant women having daily symptoms from the gas go to 72 percent experiencing some level of heartburn severity in the third trimester. What some of the findings seem to have in common is that there is an increase in the number of women who have daily heartburn the further into gestation the pregnancy progresses.
No one is exactly sure why this phenomenon occurs but there are plenty of theories, all of which are plausible. For one, the shifting levels of hormones during pregnancy, particularly progesterone and estrogen, can cause changes in the way the GI tract operates. As an example, progesterone in higher levels in the pregnant body causes the GI tract to relax, not a good thing for the LES. Also, progesterone, paired with the constipating effect of the iron-rich prenatal vitamins prescribed during pregnancy, can lead to decreased bowel motility and an increase in time that food takes to leave the stomach, known factors for a rip-roaring case of acid reflux.
Another possible reason for the increase in heartburn among pregnant women can be found in the logistics of carrying a growing fetus inside the body. As the fetus develops from a tiny, nearly weightless speck to that of a multi-pound, full-sized being, the body must adjust to accommodate sharing the living space, so to speak. In many ways, it is like moving a second person into a one-person studio apartment. Just as you would have to move some furniture around to adjust to the extra resident, the female body has to slightly displace some organs in the normal anatomy to accommodate the fetus. As the fetus grows, it presses the intestines upward, placing greater pressure on the stomach. That force alone can cause the LES to open and stomach contents to reflux into the esophagus. Aside from the constant pressure of the growing fetus, additional, transient jabs can occur when the fetus licks or moves up sharply.