FSH is needed to stimulate the growth of Graafian follicles (egg). The lowest concentration would be around day 20 or 14-16 days after the first day of your period. The reason it drops at this time is because the follicles (egg) necessary for the cycle has already been matured for fertilization during ovulation. When the FSH levels are the lowest is when you are most fertile. When the FSH increases it is recruiting follicles for the next cycle, therefore flushing out the old follicles through the blood flow during menstruation.