Progesterone is a natural hormone made by the body and has many important functions which are not provided by synthetic progestins.
Progesterone is necessary for the survival and development of embryo and throughout gestation. On the other hand, the use of progestin (medroxyprogesterone acetate and megastrol acetate) in early pregnancy may increase the risk of early abortion. They are not found in any living forms.
Synthetic progesterone binds with the same receptor sites as natural hormones however the synthetic molecule will convey a different message to the cell. This explains the alarming array of side effects, adverse reactions and contraindications.
The use of natural hormones is safer and more appropriate for the body.