Study Reveals Factors Behind Age of Girls' First Period

24 November 2010 - Lifestyle and health factors starting from before birth may affect the age at which a girl begins her periods, a large-scale study led by the ICR has found.

Danielle Morris from the ICR’s Section of Epidemiology led the analysis of data from 81,000 UK women taking part in the Breakthrough Generations Study, a comprehensive study into the causes of breast cancer.

Age at menarche (when periods begin) has long been established as a risk factor for breast cancer, possibly because these women are exposed to female sex hormones for a longer period of time. Disease risk gradually increases with progressively younger age at menarche and older age at menopause.

The study found average age of menarche was 12.7 years, with five per cent of participants starting before the age of 11 and 10 per cent at age 15 or older.

Mothers who smoke or suffer pre-eclampsia during pregnancy were more likely to have daughters who menstruated early. Other factors linked to early menstruation included having a low birth weight, being a singleton (not a twin), not being breastfed, having fewer siblings, having an early birth order (being an older sibling) and being non-white.

The study also confirmed that girls who were heavier or taller than other children at age seven and exercised little as children were more likely to start their first period at a young age.

When the scientists examined several factors in combination they found that, for example, a non-white girl who was heavier and taller than her peers at age seven could begin menstruating nearly two years earlier on average than a white girl of average height and weight.