It may seem completely arbitrary, but according to a new UK study it is genetics that are the key to the age at which girls start their periods.

If you ask around amongst your female friends the range at which their periods began can be quite wide from as young as 9 to the late teens.  Now it seems that genetic makeup explains more than half of the variation between UK women’s ages at first period, according to a study of almost 26,000 women.   The age at which girls periods begin is known to run in families but the balance of genetic and environmental influences on this has been unclear.

Scientists at The Institute of Cancer Research (ICR) analysed data from women participating in the Breakthrough Generations Study – a major UK-wide investigation into the causes of breast cancer – who had at least one other female relative also taking part.  What they found was that a woman’s age when beginning menstruation was significantly correlated with that of her relatives. For each 12 month delay in the age at which an older sister, mother or paternal aunt began their periods, there was a delay of around three months on average for the younger relative.  When the relative was a maternal grandmother or maternal aunt the delay in the younger relative was about 1.5 months and, not surprisingly, the age at which period began is also strongly correlated between twins, particularly identical twins.

Why does it matter?

The age at which menstruation begins is important because it has been linked to the risk of a number of chronic diseases including breast cancer.  This risk gradually increases with progressively younger age at menstruation and older age at menopause, possibly because women are exposed to female sex hormones for a longer period of time. Each two year delay in menstruation is associated with an estimated 10 per cent reduction in the relative risk of breast cancer.   This is important for the following reasons:

•           Breast cancer is the most commonly diagnosed cancer in the UK – nearly 46,000 women and around 300 men are diagnosed every year

•           Breast cancer accounts for nearly one in three of all female cancers

•           More than 1,000 women die of breast cancer every month in the UK

If you know that these risk factors apply to you then it is important that you take all possible precautions to ensure your risk is lowered as much as it can be.  The good news is that more women than ever in the UK are surviving breast cancer thanks to better awareness, better treatments and better screening.  This includes having a sensible anti-estrogenic diet and   minimising exposure to it by avoiding where possible sources such as the contraceptive pill and HRT.