A young girl's fertility starts when her periods commence towards the end of puberty (called menarche). The timing varies between individual girls, often similar to when their mother or grandmother started their periods. Menarche can happen any time between 9 and 18 years of age, but usually between 12 to 15 years. Women have the potential to bear children from menarche to menopause, generally up to 45 and 55 years of age, averaging between 47 to 51 years.
During a woman's fertile years her reproductive hormones are at their highest levels, changing on generally a monthly basis to prepare her body for the possibility of conceiving a baby, about 300 to 500 times in her life. Hormonal interactions cause her uterus to build up a rich lining each month, in preparation for a fertilised egg to implant and grow. But if an egg is not fertilised, the thick lining of the uterus is shed a couple of weeks later as a period.
A woman's menstrual cycle is often explained on a time scale of 28 days, which is the most common length of time between periods. However, some women experience regular variations (or infrequent fluctuations) ranging from as short as 20 days to as long as 40 days. Shorter and longer cycles are very normal and a woman is still fertile, they are just regarded as individual differences.
Some women find their cycles are consistently longer or shorter (eg. always around 35 days, or always around 24 days), while others notice fluctuations from month to month, or year to year and at different times of their lives. For example, 26 days one month and 31 days the next. Puberty and menopause are typical times to experience irregular periods, as the body goes through these major transitional phases.
Changes in period patterns can also happen as you grow older.