A woman’s ovulation period is from the first day of her menstrual cycle to the fourteenth day of her menstrual cycle, but if the menstrual cycle is delayed, ovulation is also delayed. When ovulation occurs, the ovaries start to secrete a small amount of estrogen and the follicles begin to develop; on the seventh day of menstruation, the follicles begin to secrete a rapidly increasing amount of estrogen; before ovulation, a peak can be reached in the body, when the hypothalamus is also secreting follicle-stimulating hormone, which is capable of gradually increasing the size of the follicles. The follicle-stimulating hormone is then able to encourage the follicles to enlarge gradually. Clinically, when the follicle enlarges to more than 18mm, it is called the dominant follicle, and this is the time to ovulate.