<span class="citations"><p>Nejib M. Nasraoui, Alain Piasco, "Derivatives of 19-nor progesterone; process for producing them and the pharmaceutical compositions incorporating them." U.S. Patent US5223492, issued May, 1971.</p>
</span><a href="https://www.google.com/?tbm=pts#q=5223492&amp;tbm=pts" rel="noopener" target="_blank">US5223492</a>