The Women's Health Initiative study correctly randomized women with a uterus into groups receiving conjugated equine estrogen plus MPA or placebo, and women without a uterus into groups receiving conjugated equine estrogen or placebo.

“The study took um two groups of women women who had a uterus and women who didn't have a uterus and randomized each of those groups into two separate groups treatment versus placebo why was that done well it was well understood by then as it Still Remains that 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 so to this day we know this and we do this um so if you had a uterus you were put into a group where the treatment group was given conjugated equine estrogen and MPA so that's uh that's estrogen taken from horse urine and a synthetic progestin and the treatment and the placebo group was just given a placebo and then in the other group The no uterus group they were just given conjugated equin estrogen versus placebo they didn't have to be given the mpa the synthetic estrogen”