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BACKGROUND: In normo-ovulatory infertile women undergoing mild ovarian stimulation out of IVF, FSH stimulation regimen must be carefully adjusted to control the number of recruited follicles and to prevent multiple pregnancies. The aim of this prospective study was to assess the effect of the timing of FSH administration (fixed dose and duration) on the number of large follicles. METHODS: Women were prospectively randomized by means of sealed envelopes to receive daily 112.5 IU recombinant FSH (rFSH), either from cycle day (CD) 2–6 (Group A) or from CD 7–11 (Group B). Hormonal measurements and follicular ultrasound assessments were performed on CD 2, 7 and 12. RESULTS: On CD 12, the development rate of exactly two follicles ≥14 mm in diameter was significantly lower in Group A than in Group B (4% of women versus 42%, P = 0.002). Although the pattern of serum estradiol (E2) concentrations in Group A displayed a plateau from CD 7, the cancellation rate for overstimulation (more than three follicles ≥14 mm in diameter) was significantly increased (P = 0.009). CONCLUSIONS: Preventing the closure of the FSH window by mid to late follicular phase FSH administration better fulfils the objective of obtaining a limited number of large follicles than surpassing the FSH threshold by an early administration. [ABSTRACT FROM AUTHOR]