Final answer:
During pregnancy, increased progesterone levels lead to a decrease in LES tone. The decrease of LES tone can cause heartburn or acid reflux. As progesterone levels drop and estrogen levels rise closer to labor, uterine contractility is increased, preparing the body for birth.
Step-by-step explanation:
During pregnancy, the tone of the lower esophageal sphincter (LES) decreases due to increased levels of progesterone. Progesterone is known to inhibit uterine contractions throughout the early stages of pregnancy to prevent premature labor. As the pregnancy progresses into the third trimester, the concentration of progesterone begins to plateau and subsequently decrease, while estrogen levels rise. The decreasing LES tone is associated with the relaxation effects of progesterone, which, in the case of the LES, can lead to symptoms such as heartburn or acid reflux commonly experienced during pregnancy.
Furthermore, the drop in progesterone towards the last weeks of pregnancy plays a crucial role in initiating labor by decreasing the inhibition of uterine contractility. This physiological change makes the myometrium more sensitive to contractions as estrogen continues to rise. As labor approaches, estrogen facilitates an increase in oxytocin receptors on the uterine wall, and along with signals from the stretch of the cervix, labor is initiated with uterine contractions.