Breasts are getting bigger, but also deadlier.

The average U.S. bra size has grown from a 34B to a 36C in just one generation, but Florence Williams, the author of a new book Breasts: A Natural and Unnatural History, found that the trend towards bustier breasts takes a toll on women's health.

Not only are women's breasts today larger than they've ever been, so are their waistlines.

In her book, Williams illustrates a number of disturbing trends that may contribute the high rate of breast cancer in the U.S., like weight gain and early onset of puberty, both of which have been linked to an elevated risk of breast cancer.

Girls today are hitting puberty earlier than ever before, and 15 percent of all American girls begin to grow breasts at just seven-years-old, according to a 2010 study in pediatrics.

Bigger breasts also put women at a greater exposure for pollutants because the body stores toxic chemicals in fatty tissue.

Therefore the bigger the cup size, the greater concentration of fatty tissue to store chemicals like mercury and Polychlorinated biphenyls (PCBs), an oily, odorless and tasteless industrial chemical used in pesticides and paint.

Not only have these chemicals been shown to remain in body fat and organs for months after being exposed, they have also been found to accumulate in breast milk and transfer to infants during breastfeeding.

What's worse is that PCBs have been shown to both inhibit and mimic estradiol, a major sex hormone in women, and scientists have found that the imitation of the estrogen compound supports the development of estrogen-dependent breast cancer cells, uterine and cervical cancer cells.

"Breast-feeding, it turns out, is a very efficient way to transfer our society's industrial flotsam to the next generation," Williams writes. "Our breasts soak up pollution. … Breasts carry the burden of the mistakes we have made."

Williams had a sample of her own breast milk analyzed while nursing her second child, and found that it contained levels of perchlorate, an ingredient in jet fuel and chemical flame retardants, that were 10 to 100 times higher than in European women. 

While she supports breastfeeding because of its benefits for children's brain, body and immune system development, she warns that many man-made toxins will remain in our bodies and in our children's bodies for long enough that today's baby girls will transmit the toxins to their own children.

"What happens in our environment is reflected in our breasts," she said, according to USA Today. "If we really care about human health, we need to care about our planet."

While Williams couldn't identify exactly which of the many different contaminants are responsible for breast cancer or health issues relating to breasts, she told ABC news that her personal experience was a "great way to tell the story first-person".

Not only have cancer rates doubled since the 1940s, today one in eight women are at risk of developing breast cancer, Williams said.

"Our bodies are intimately connected to the world around us," she wrote. "If we live in an environment filled with pollution, these things will and do affect our health."