Abstract: The 15?,16?-methylene-17-hydroxy-19-nor-17-pregna-4-en-3-one-21-carboxylic acid ?-lactone derivatives of the present invention possess progestational efficacy. They have the general chemical formula I, in which Z is selected from the group comprising an oxygen atom, two hydrogen atoms, NOR and NNHSO2R, where R is hydrogen, C1-C10-alkyl, aryl or C7-C20-aralkyl, R4 is hydrogen or halogen, and moreover either: R6a, R6b together form methylene or 1,2-ethanediyl or R6a is hydrogen and R6b is selected from the group comprising hydrogen, C1-C10-alkyl, C2-C10-alkenyl or C2-C10-alkynyl, and R7 is selected from the group comprising hydrogen, C1-C10-alkyl, C3-C6-cycloalkyl, C2-C10-alkenyl or C2-C10-alkynyl, or: R6a is hydrogen and R6b and R7, together, denote a bond, an oxygen or form methylene, R18 represents hydrogen or C1-C3-alkyl and in addition include their solvates, hydrates, stereoisomers and salts.