Having more than 11 moles on one arm may be a sign that you’re at higher-than-average risk of skin cancer, according to new research.

More than 100 indicates five times the normal risk according to a study published in the British Journal of Dermatology , which studied data from 3,000 female twins.

Melanoma, a type of skin cancer, develops from abnormal moles, so the greater the number the greater the chances one could turn cancerous.

Researchers from King’s College London collected information on skin type, freckles and moles over eight years and came up with the rule-of-thumb guide that could help doctors in a a paper called "Melanoma and naevi with a globular pattern: confocal microscopy as an aid for diagnostic differentiation".

Females with more than seven moles on their right arm had nine times the risk of having more than 50 on their whole body. Those with more than 11 on their right arm were more likely to have more than 100 on their body in total, meaning they were at a higher risk of developing a melanoma.

Lead author Simone Ribero said: “The findings could allow GPs to more accurately estimate the total number of moles in a patient extremely quickly via an easily accessible body part.”

Dr Claire Knight, at Cancer Research UK , said it was important to understand fewer than half of melanomas develop from existing moles.

She said: “It’s important to know what’s normal for your skin and to tell your doctor about any change in the size, shape, colour or feel of a mole or a normal patch of skin. And don’t just look at your arms – melanoma can develop anywhere on the body, and is most common on the trunk in men and the legs in women.”

The problem, for many, is deciding whether a brown blemish is a mole or a freckle. The science behind them is easy – a mole is a circular brown area made up of cells which produce the colour. They are not linked to sun exposure although excessive exposure could turn them malignant.

Cancer warning signs

Warning signs to look out for include uneven colouring, ragged edges, bleeding, itching and enlargement.

The NHS says :

If you have a lot of moles, it’s important to take extra care in the sun. Although it’s not always possible to prevent melanoma, avoiding overexposure to UV light can reduce your chances of developing it.

You can help protect yourself from sun damage if you:

  • Stay in the shade when the sun is at its strongest (between 11am and 3pm).
  • Cover up with clothes, a wide-brimmed hat and sunglasses.
  • Use a high-factor sunscreen (minimum SPF15) and reapply it regularly, particularly after swimming.
  • Avoid using sunlamps or sunbeds because they give out UV rays.