Risk: Woman thin at age seven are more likely to get cancer.
Skinny girls are more likely to develop breast cancer in later life, research has found.
Females underweight at the age of seven are at greater risk of the disease when they get older than those who are larger in size.
Scientists at Karolinska Institute in Stockholm also found that girls who were slightly overweight at a young age were less likely to develop particularly aggressive types of tumours which are very difficult to treat.
The research, published today in the Breast Cancer Research journal, could pave the way for old childhood photos being used as a means of estimating a woman's risk of breast cancer.
The scientists studied 6,000 women in Sweden - half of whom were breast cancer patients - and split them into three groups depending on whether they were ‘lean’, ‘medium’ or ‘large’ build when they were seven years old.
They used photographs and their own memories as a basis.
Surprisingly they found that women who were bigger when younger were less likely to develop the disease in the menopause.
Previous research has found that obese females are much more prone to breast cancer. They are also 50 per cent more likely to die from the disease.
The scientists do not know why skinny girls are more likely to develop breast cancer.
They say their findings could have important implications in determining a woman's risk.
Jingmei Li, who lead the research, said: ‘It appears counterintuitive that a large body size during childhood can reduce breast cancer risk, because a large birth weight and a high adult BMI have been shown to otherwise elevate breast cancer risk.
‘There remain unanswered questions on mechanisms driving this protective effect.’
She added: ‘Given the strength of the associations, and the ease of retrieval of information on childhood shape from old photographs, childhood body size is potentially useful for building breast cancer risk or prognosis models.’
The study also showed that larger girls were less likely to develop what are known as ‘oestrogen receptor negative’ tumours, one of the most deadly forms of the disease.
Breast cancer is the most common form of cancer in women and up to 1 in 9 will get the disease at some point in their lives.
Up to 46,000 new cases are diagnosed each year.