In most cases, the follicles will ovulate when they reach 18mm or more. After a woman has her period, the follicles inside the ovaries start to develop, which is slow and small at first, only 3-5mm, but as the hormone levels in a woman’s body slowly increase, the follicles will slowly grow. At the time of ovulation, most women will find that the follicles can grow to more than 18mm, and there will be eggs inside such large follicles. When the eggs are discharged, women may have follicular fluid discharged, and at this time, women will feel pain in the belly, waist pain, and thin leucorrhea.