What hormone is primarily responsible for the changes in the endometrium?

Progesterone is the hormone primarily responsible for the changes in the endometrium, particularly during the latter half of the menstrual cycle, known as the luteal phase. After ovulation, the corpus luteum, which forms from the ruptured follicle, secretes progesterone, leading to significant modifications in the endometrial lining. Progesterone promotes the thickening of the endometrium, preparing it for potential implantation of a fertilized egg. This hormone ensures that the endometrium is rich in blood vessels and nutrients, creating a suitable environment for embryonic development if fertilization occurs.

If pregnancy does not take place, progesterone levels drop, leading to the shedding of the endometrial lining during menstruation. Thus, it plays a critical role in regulating the menstrual cycle and maintaining early pregnancy. Estrogen does contribute to the growth and repair of the endometrium earlier in the cycle, but it is progesterone that is pivotal for the changes that occur after ovulation. Other hormones like testosterone and follicle-stimulating hormone do not have a primary role in altering the endometrial layer.