Progesterone prepares the endometrium for the potential of pregnancy after ovulation. It triggers the lining to thicken to accept a fertilized egg. It also prohibits the muscle contractions in the uterus that would cause the body to reject an egg.

Does progesterone thin endometrial lining?

Taking progesterone will cause the lining to shed and prevent it from building up again. It often will cause vaginal bleeding.

How does progesterone cause endometrial thinning?

Progesterone prepares the endometrium to receive and nourish a fertilized egg. If pregnancy does not occur, estrogen and progesterone levels decrease. The decrease in progesterone triggers menstruation, or shedding of the lining. Once the lining is completely shed, a new menstrual cycle begins.