Researchers at a commercial DNA testing service say they have found a handful of genes that help determine whether a woman spends her life as an A cup or a D.
Those genes might also be tied, they say, to a woman's risk of breast cancer.
"There are surprising connections between some of the genes involved in determining breast size and the genes involved in breast cancer," lead author Nick Eriksson, a researcher with the California-based personal genomics company 23andMe, told The Huffington Post.
In a study published in the journal BMC Medical Genetics, Eriksson and his colleagues analyzed data from more than 16,000 female customers who had previously had their genetic makeup examined. The researchers were looking for single nucleotide polymorphisms, or SNPs, which are variations in DNA that occur when a single nucleotide in a sequence is altered. Some SNPs have no impact on cell function; others can predispose people to certain traits or illnesses.
After comparing the women's genetic data with information they provided about their bra size, researchers identified seven SNPs as "significantly associated" with breast size, three of which have been previously linked to breast cancer risk.
The findings suggest that a woman's cup size and her risk for developing breast cancer could be connected. However, even the researchers are quick to admit that the connection should be regarded as preliminary at best.
"It's fair to say that the link is a bit uncertain, and based on current knowledge, it's not a strong risk factor," said Eriksson. He suggested that one possible, albeit oversimplified, explanation for the findings could be that larger breast size means more cells that could become cancerous.
But, he added, "part of the complication is that obesity also plays a complicated part in breast cancer risk."
Indeed, breast cancer specialist Dr. Edith Perez, deputy director at large at the Mayo Clinic Cancer Center (who was not involved in the 23andMe research), said that a major limitation of the new study is that researchers did not have complete information about the participants' weight, which in many cases can directly influence a woman's breast size. Obesity has also been shown to increase breast cancer risk, particularly after menopause.
In addition, Perez said, the researchers did not control for other factors that can increase risk, such as alcohol use and breast density. The latter trait is largely inherited and has been much more definitively linked to cancer risk than breast size has. Studies have suggested that women with dense breasts are up to five times more likely to develop breast cancer, for reasons that are not yet fully understood.
"The way I look at it is that it's an interesting finding, but I do not think it will have a big impact on the way we stratify for risk for breast cancer," said Perez, adding that the fact this is a commercial company's study (as opposed to an independent data analysis) should be taken into consideration. She said she hoped future research would look at factors that women might be able to change or influence in order to cut their risk for breast cancer, which is currently the second-most common cancer among women in the United States. (Skin cancer is the first.)
Eriksson echoed that the new findings may have more implications for researchers interested in the possible connection between breast size and cancer risk than for women directly. For now, breast size is not a "major factor," he explained, and certainly nowhere near as significant as obesity or breast density.
The biggest takeaway from the new study may be that when it comes to cup size, a lot depends on the genes.
"Breast size is definitely heritable," Eriksson said. "But unlike height, where you can look at both parents and get some idea how tall you will be, you have much less data for breast size. A young woman can look at her mom. However, she won't get the same insights by looking at her dad for his genetic contribution."