PARIS, France (AFP) -- Girls who are thin at the age of seven are at higher risk of breast cancer after menopause than chubbier counterparts and more vulnerable to a worse type of tumour, according to a Swedish study published on Wednesday.

The surprising conclusion is made by Karolinska Institutet researchers who looked at data from 2,818 Swedish breast cancer patients and 3,111 healthy counterparts.

"Large body type at age seven years was associated with a decreased risk of post-menopausal breast cancer," said lead researcher Jingmei Li.

"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 elevate breast cancer," Li added.

"There remain unanswered questions on mechanisms driving this protective effect," he said.

BMI means body mass index, a measure of fat.

A girl's BMI at the age of seven also drove the risk of so-called oestrogen receptive negative tumours, where the outcome is often less favourable than other cancer types, said the study.

The paper is published online in an open-access, peer-reviewed journal, Breast Cancer Research.