19-Norprogesterone, also known as 19-norpregn-4-ene-3,20-dione, is a steroidal progestin and close analogue of the sex hormone progesterone, lacking only the C19 methyl group of that molecule. It was first synthesized in 1944 in the form of a mixture that also included unnatural stereoisomers of progesterone, and this mixture was found to be at least equivalent to progesterone in terms of progestogenic activity. Subsequent investigations revealed that 17-isoprogesterone and 14-iso-17-isoprogesterone are devoid of progestogenic activity. 19-Norprogesterone was resynthesized in 1951 with an improved method, and was confirmed to be the component of the mixture synthesized in 1944 that was responsible for its progestogenic activity. In 1953, a paper was published showing that 19-norprogesterone possessed 4- to 8-fold the activity of progesterone in the Clauberg assay in rabbits, and at the time of this discovery, 19-norprogesterone was the most potent progestogen known.