In the 1960s, estrogen replacement therapy likely did not include progesterone, potentially leading to endometrial hyperplasia.

“if we go back into probably gosh the 1960s when the idea first came along to Physicians to replace estrogen in a menopausal woman I think they were just probably using estrogen alone correct and they didn't understand the role of progesterone as opposition to that to prevent endometrial hyperplasia”