Final maturation of follicles has, in connection with ovarian stimulation and infertility treatment, traditionally been achieved by the administration of a human chorionic gonadotropin (hCG) bolus trigger of 5,000 to 10,000 IU. This trigger serves two purposes: induce oocyte maturation; and serve as luteal phase support owing to its long half-life. It now appears that the hCG bolus trigger is unable to support both these two purposes optimally. In particular, after an hCG trigger, the early luteal phase is hormonally abnormal and different from conditions observed in the natural menstrual cycle: the timing of the initiation of hCG and progesterone rise is much faster after an hCG trigger than in a natural menstrual cycle; the maximal concentrations of hCG and progesterone considerably exceed those naturally observed; and the timing of the peak progesterone concentration after an hCG trigger is advanced several days compared with the natural cycle. Furthermore, the hCG trigger without any follicle-stimulating hormone activity may induce oocyte maturation less efficiently than the combined luteinizing hormone and follicle-stimulating hormone surge normally seen. Collectively, the endometrium is likely to be advanced after an hCG trigger, and the implantation potential is probably not optimal. The precise effect on pregnancy rates after the different progressions of hCG and progesterone concentrations during the early luteal phase has not yet been determined, but more individualized methods using more physiological approaches are likely to improve reproductive outcomes.