The ovarian cycle starts on the first day of the menstrual cycle. The menstrual cycle begins on the first day of the periods - which is the first day of bleeding. At this time, the levels of all the hormones - estrogen, progesterone , FSH and LH - which are primarily responsible for maintaining menstruation, are at the baseline levels.
The low level of estrogen in the blood at this time, stimulates the pituitary gland in the brain to start producing Follicle Stimulating Hormone (FSH). The level of FSH rises and stimulates the Graafian follicles in the ovaries to develop. This indicates the start of the ovarian cycle in menstruation.