Final answer:
FSH reaches its maximum concentration just before the mid-cycle, around day 12 to day 14, and is followed by a rapid decline due to high estrogen levels which cause ovulation and the transition from the follicular to the luteal phase of the menstrual cycle.
Step-by-step explanation:
Follicle-stimulating hormone (FSH) reaches its maximum concentration just before it begins to decrease as a result of negative feedback from high estrogen levels, typically around day 12 to day 14 in a 28-day menstrual cycle. Just prior to the middle of the cycle, elevated estrogen production from the dominant follicle switches the feedback to positive, stimulating a rapid rise and subsequent decline of both FSH and luteinizing hormone (LH), particularly the LH surge. This LH surge triggers ovulation, causing the most mature follicle to rupture and release its egg, marking the end of the follicular phase and the beginning of the ovulatory phase.
During this peak period of FSH and the LH surge, all but one of the developing tertiary follicles undergo atresia, due to the decrease in FSH levels. The surviving dominant follicle is able to continue maturing due to its heightened sensitivity to FSH and strong estrogenic feedback, which is now positive. When ovulation occurs around day 14, the ovarian cycle moves from the follicular to the luteal phase, with the ruptured follicle forming the corpus luteum which produces progesterone, playing an important role in the subsequent secretory phase of the menstrual cycle where the uterine lining is prepared for potential implantation of a fertilized egg.