杏吧原创

Big baby girls are likelier to develop breast cancer as adults than average-sized infants, according to a study of more than 22,000 women with the disease.

A 500-gram increase in birthweight increases a child鈥檚 risk of developing breast cancer by 7%, while a 2-centimetre increase in height boosts the odds by 10%, says , an epidemiologist at the London School of Hygiene and Tropical Medicine who headed the international study.

She and colleagues reanalysed data from dozens of clinical studies performed over the last 20 years.

Many of these hinted at a connection between birth size and breast tumours, but few of the studies included enough patients to determine whether the link was real or a statistical fluke, as well as the magnitude of the connection.

鈥淚nstead of having just a couple of hundred breast cancer patients in each study, we have 22,000, and that makes a huge difference,鈥� she says. Her team鈥檚 study also pooled data on more than 600,000 women without breast cancer.

Small increase

For most of these patients, the researchers had only measurements of birth weight gathered from their mothers. 鈥淣one of us knows our birthweight, we have to ask our mums,鈥� she says. Among these patients the link between birthweight and breast cancer risk was small.

However, more consistent measurements, gathered by doctors and nurses at the time of birth, bolstered the connection between birth weight and adult breast cancers.

A girl born weighing more than 4 kg was 12% more likely to develop a tumour than a child who weighed between 3 and 3.5 kg at birth.

All told, larger babies might account for 5% of all breast cancers, dos Santos Silva says. 鈥淚t is modest, but not very different from the relationship between breast cancer and alcohol,鈥� she says.

In 2007, 548,000 women died of breast cancer globally, according to the World Health Organization. In the US, 12.7% of women develop breast cancer.

Growth factor

High maternal levels of an oestrogen hormone called estradiol might be one explanation for the connection, says dos Santos Silva. Other growth hormones, or even overactive stem cells, could play a role, but 鈥渨e can only speculate鈥�, she says. 鈥淲e really don鈥檛 know.鈥�

If oestrogen is responsible for the increased risk of breast cancer, large baby girls might be more likely to develop breast cancers that feed off of the hormone. Testing for that would be interesting, says dos Santos Silva.

Oncologists typically classify breast cancers by their ability to respond to oestrogen, and one pharmaceutical company has recently developed a drug that preferentially targets oestrogen-fed tumours.

For now, the connection between birth size and breast cancer remains a scientific debate, not a medical one. 鈥淲e can鈥檛 really say we are going to shift the distribution of birth weight,鈥� she says. 鈥淪mall babies have their own problems.鈥�

This study 鈥渞adically changes the way we look at the origins of breast cancer鈥�, says , an epidemiologist at the Harvard School of Public Health.

Genetic and environmental factors don鈥檛 entirely account for breast cancer rates, and a woman鈥檚 risk of developing the disease probably starts in the womb, he says.

Journal reference:

Cancer 鈥� Learn more about one of the world鈥檚 biggest killers in our comprehensive special report.