During a normal menstrual cycle, the levels of the hormones estrogen and progesterone increase and decrease in a regular pattern. Ovulation occurs in the middle of the cycle, and menstruation occurs about 2 weeks later.During perimenopause, hormone levels may not follow this regular pattern.enopause is defined as the absence of menstrual periods for 1 year. The average age of menopause is 51 years, but the normal range is 45 years to 55 years. The years leading up to this point are called perimenopause.