Un-opposed estrogen in women with a uterus increases the risk of endometrial hyperplasia, dysplasia, and cancer.

“in women with a uterus failure to give progesterone with estrogen increases endometrial hyperplasia so if you take a woman with a uterus and you just give her estrogen but there's no progesterone her endometrial lining will thicken will thicken will thicken and as the endometrial lining gets thicker so too goes the risk of hyperplasia and ultimately what's called dysplasia which can lead to cancer in other words un opposed estrogen will increase the risk of endometrial cancer”