Follow-up follicle monitoring in the same cycle uses transvaginal ultrasounds to track the growth and maturation of ovarian follicles, starting with a baseline scan on days 2-3 of the menstrual cycle and continuing with frequent scans (every 1-3 days, then daily) until a dominant follicle reaches 18-22mm and is ready for ovulation. 

This monitoring allows doctors to pinpoint the fertile window for natural conception, time fertility treatments like IUI or IVF, or administer a trigger shot at the optimal time to induce ovulation.