Menstrual cycle is regulated by hormones of endocrine system and regular menstrual period occurs every month during the fertile years of a women's life. A woman's first menstruation starts typically around age 12 and termed as menarche. Woman's reproductive phase ends between the ages between 45 and 55 and is known as the menopause.
A variation of two to three days between menstrual cycles is normal and for some women, menstrual cycle is repeated almost regularly in every 28 days. Although the period of the menstrual cycle can vary from 21 to 35 days, the average cycle lasts for 28 days.
Menstrual cycles are counted from the first day of menstrual bleeding and the menstrual cycle is broadly divided into three phases: the follicular phase (menstruation - controlled by estradiol), ovulation (proliferative phase - controlled by luteinizing hormone (LH) and follicle-stimulating hormone (FSH)), and the luteal phase (secretory phase - the main hormone associated with this stage is progesterone).
Menstrual bleeding normally lasts between two and seven days, with the average being five days and periods last for longer than seven days are generally considered as abnormal.
According to American Congress of Obstetricians and Gynecologists (ACOG), menstrual cycles longer than 35 days or shorter than 21 days are abnormal.
Normal variation between the lengths of 2 cycles is two to three days. Cycle length variation of 8-20 days between two cycles is considered as moderately irregular and variation of 21 days or more is considered very irregular.
Many of these abnormal patterns are caused by age, hormonal changes or by physical and mental statuses. Explanations about pathological conditions associated with abnormal menstrual pattern are beyond the scope of this web site. It is advisable to check with your doctor, if you experience any of these problems.