The present invention relates to crystalline forms of 11 β - (4-acetylphenyl) -20,20,21,21,21-pentafluor-17-hydroxy-19-nor-17α -pregna-4,9-dien-3-one . In particular, the invention relates to two crystalline ansolvate / anhydrate forms of this compound, polymorphs I and II. However, the present invention also relates to crystalline solvates, for example,methanol and ethanol solvates of 11 β - (4-acetylphenyl) -20,20,21,21,21-pentafluor-17-hydroxy-19-nor-17α-pregna-4,9-dien-3, one, as precursors for the preparation of these two polymorphs I and II. Processes are described for the preparation of polymorph I by means of displacement crystallization or by means of extraction. The choice of the last solvent, before the ansolvate formation can take place, on the basis of the differences in the purification ratios of the individual solvates of 11 β - (4, acetylphenyl) -20,20,21,21,21-pentafluor -17-hydroxy-19-nor-17 α -pregna-4,9-dien-3-one is described. Polymorph I, according to the invention, is particularly suitable for the preparation of drugs.
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