The time interval between menstrual periods is determined by the rate and quality of the growth and development of the follicle (fluid filled structure containing and supporting the egg) and the duration of the luteal phase (period of time after ovulation). Women who have normal ovulatory cycles generally have intermenstrual cycle lengths ranging between 25-35 days. Only 15% of cycles in reproductive aged women are actually 28 days in interval.
There are a variety of factors that can temporarily or chronically alter this time interval. Environmental factors such as stress, strenuous exercise and extremes of diet (anorexia, bulimia) can inhibit ovulation and cause a delayed or missed period. Hormonal imbalances, including thyroid disorders, excess prolactin, excess androgens (male hormones ex/polycystic ovary syndrome) can do the same. These conditions can be treated with medication. As a woman ages and the number and quality of eggs declines, the time from one period to the next declines at first, and then starts lengthening again eventually leading to menopause. A thyroid abnormality or a progesterone deficiency (luteal phase defect) also can cause a shorter intermenstrual cycle length.