Each month one egg ripens and drops from the ovary into the uterus. The spot where the egg has left the ovary then produces a second hormone called progesterone. So a woman has the first hormone, estrogen throughout most of her cycle and the second hormone, progesterone, only after she releases an egg for the last part of her cycle. Women who have estrogen but no progesterone are at high risk for uterine cancer. Estrogen stimulates the uterus to grow.
Progesterone stops the stimulation.
Women with only estrogen have a uterus that is stimulated all the time, which can lead to uncontrolled growth which is cancer. To prevent uterine cancer, doctors are supposed to prescribe progesterone to all women who lack that hormone. I am appalled by the outlandish claims made by people selling natural progesterone creams. They have not been shown to be safer and more effective than progesterone pills. It is not true that natural progesterone cream turns fibrocystic breasts back to normal, or prevents hair loss.