A woman who has had a hysterectomy should not take the combination. Plain conjugated estrogen is all that is needed in that situation. It is the estrogen alone that appears to account for all the benefits of HRT, including the slowing or reversal of osteoporosis, the improvement of cardiac risk, and the maintenance of skin and mucus membranes, especially in the vagina. Studies are also now appearing that seem to show that HRT reduces the risk of Alzheimer's disease by about 40 to 50 percent. But it is almost certainly synthesized from a naturally occurring steroid molecule, which is chemically manipulated to make the desired hormone.
Many medicines are the result of chemical manipulation of naturally occurring molecules.
Certainly the yeast that produces it is natural. The PDR states that Premarin, the Prem in Prempro, is made from natural sources. Medroxyprogesterone is synthesized from chemical precursors. My next problem stems from the implication of many of the questions that natural is better. The problem I have with many natural products is that we often don't know what the ingredients of the product are, to say nothing about its effectiveness and safety.