Method of treating or preventing infertility in a female mammal and pharmaceutical kit for use in such method

The present invention relates to a method of treating infertility in a female mammal that involves controlled ovarian hyperstimulation, the method comprising administering to the female mammal a combination of (i) an FSH substance in an amount effective to stimulate follicular development and (ii) a steroid in an effective amount to inhibit or suppress the secretion of luteinising hormone, the steroid being selected from the group consisting of: substances represented by the following formula (I) in which formula R1, R2, R3, R4 independently are a hydrogen atom, a hydroxyl group or an alkoxy group with 1 -5 carbon atoms; each of R5, R6,R7 is a hydroxyl group; no more than 3 of R1, R2, R3, R4 are hydrogen atoms; derivatives of the aforementioned steroid substances; and mixtures of one or more of the aforementioned substances and/or derivatives.