We know for certain that DES, when given to normal women, at best did nothing and at worst increased reproductive wastage (miscarriages) as demonstrated in the clinical trials by Dieckman and others.
There was never an adequate large-scale randomized clinical trial of that established DES with problem pregnancies. Small, controlled studies concluded that DES was an ineffective treatment for threatened abortion but failed to achieve public notice or to generate further research.
The research methodology used in the original studies would not be acceptable today as the exclusive method for a drug approval. The Dieckman studies suggest that in normal pregnancies, DES increases the likelihood of prematurity itself.