Which hormonal change is primarily responsible for relaxation of the sphincter tone during pregnancy?

During pregnancy, the hormone primarily responsible for the relaxation of sphincter tone is progesterone. Progesterone plays a crucial role in maintaining pregnancy and preparing the body for childbirth. It helps by relaxing smooth muscle tissue, which includes the sphincters in the gastrointestinal and urinary tracts. This relaxation of the sphincter tone can contribute to various symptoms often experienced during pregnancy, such as gastroesophageal reflux and increased urinary frequency, as the relaxed tone allows for less constricted passage through these muscular openings.

While other hormones like estrogen, testosterone, and oxytocin also play significant roles in pregnancy and childbirth, they do not specifically cause the relaxation of sphincter tone to the same extent as progesterone. Estrogen primarily supports uterine growth and blood supply; testosterone is more relevant to male development; and oxytocin is mainly involved in inducing labor and milk ejection during breastfeeding. Therefore, progesterone's particular influence on smooth muscle relaxation is the key factor in this physiological change during pregnancy.