Missed periods, or amenorrhea as it's medically called, is common and perfectly natural in young girls who are in their first couple of years of menstruation.
However, for older females, an absent period is not normal and the reasons behind it should be investigated. The main causes of a missed period are as follows:
If you place yourself under significant stress, you can affect the area of the brain that produces the hormones required for properly functioning ovaries. Substantial weight loss can have a similar effect.
An overactive thyroid gland can cause, amongst other things, absent or irregular periods.
This is perhaps the most well known cause of an absent period. If having missed a period, there is any possibility that you could be pregnant, you should probably take a pregnancy test immediately.
The menopause essentially means the end of menstration and therefore the end of monthly periods.
Polycystic Ovary Syndrome
This condition results in large numbers of ovarian cysts which can cause absent or irregular periods.