The way a girl grows during her early years may influence her risk of breast cancer, research suggests.
Tall girls may be more at risk
A Danish study of more than 117,000 women found those born chubby, but who went on to become tall and lean teenagers had the highest risk.
The biological reason for the increased risk is unclear, but the study suggests breast tissue cells may be sensitive during early periods of rapid growth.
Details are published in the New England Journal of Medicine.
Breast cancer rates have risen in recent decades - and the new study suggests that this in part might be tied to the increase in average height.
The researchers, from the Danish Epidemiology Science Centre in Copenhagen, analysed records of girls born between 1930 and 1975.
They found that a higher risk of breast cancer was associated with:
- High birth weight
- Rapid growth around the time of mammary gland development
- Being tall
- Having low body-mass-index during adolescence
They calculated that girls who were about five feet six inches tall (167.5 cm) by age 14 were 50% more likely to develop breast cancer later in life than girls who were just under five feet (152 cm) tall at the same age.
The researchers also found that newborn girls who weighed more than 8.75 pounds (4 kg) were on average 17% more likely to develop the disease later in life than those who weighed about 5.5 pounds (2.5 kg).
The lower a girl's BMI - a measure of weight relative to height - at age 14, the higher her risk of breast cancer.
And the younger a girl had her peak growth period, the higher her later risk of disease.
Writing in the journal, the researchers say: "Overall, our results provide evidence that factors influencing foetal, childhood, and adolescent growth are important independent risk factors for breast cancer in adulthood."
Researcher Professor Mads Melbye said: "Something very early on in life plays a role in risk of breast cancer. No one knows really what."
Previous research has found that tall women have an increased risk of breast cancer, and that heavy women have a higher risk of the disease after menopause.
However, Professor Melbye said science needed to focus more on growth in the early years of life if it was to pinpoint the factors that really matter.
Some experts believe that the hormones that control growth may also determine the likelihood of developing cancer. Diet may also be a factor.
The Danish team say it is possible, though not likely, that girls who reach puberty at an early age may be more at risk because they will have more menstrual cycles during their life.
In an editorial in the same journal, Dr Karin Michels and Professor Walter Willett, of Harvard University, said the study reinforces growing evidence that breast cancer may have its origins early in life.
They wrote: "An association between the risk of breast cancer and the rate of growth during adolescence has been suggested previously, but these new data are the most convincing."
More than 40,000 cases of breast cancer are diagnosed in the UK each year.
Antonia Bunnin, of the charity Breakthrough Breast Cancer, said: "Unfortunately, height or the rate at which you grow isn't something women can alter but the more we know about the causes, the greater our understanding of how to prevent and treat the disease.
"In the meantime it is vital for all women to be breast aware and visit their GP if they are concerned about changes in their breasts."
Richard Sullivan, of Cancer Research UK said: "In terms of assessing the increasing incidence of breast cancer this research may be of some value to scientists, but on an individual level, the relative risk is so small that for a woman at very low risk there is virtually no increase in risk at all."