This is because the hormonal axis involving the brain, hypothalamus, pituitary gland, and the ovaries is maturing at this time. Menstrual irregularities are fairly uncommon after the age of about 20. As menopause approaches, irregular periods again become more common. Menstrual irregularity should be taken into consideration when evaluating a woman's risk for osteoporosis. However, health problems that occurred in adolescence are often discovered only when a bone scan reveals low bone density at menopause.
Some women have irregular periods of a different sort.
This is often associated with being overweight and puts women at risk for heart disease. Women may be at risk even if they have had normal periods for most of their lives.