Dr. Philippe Grandjean, an adjunct professor at the Harvard T.H. Chan School of Public Health and a founding editor of the journal Environmental Health, often tells the story of how doctors first discovered that a mother’s exposure to mercury could harm her unborn child even without seeming to affect her.

He tells the story with the help of a 1950s-era photo of a healthy Japanese woman caring for a severely disabled son.

It’s now more or less common knowledge that developing fetuses are more sensitive to some chemicals than their mothers.

But Grandjean has moved on to challenge another assumption about chemical exposure.

Now he argues that parents’ exposure to chemicals can affect a child’s development even when the exposure happens before the child is a glimmer in the parents’ eyes.

In a paper published today in the journal Endocrinology, Grandjean and his colleagues, including some from the National Institutes of Health, offer a consensus summary of a set of papers they presented at a conference on the topic in October.

According to Maricel Maffini, Ph.D., an environmental health consultant who was not involved with the study, it offers more evidence that we should think about chemical exposure not just over our lifetimes, but across generations.

When parents are exposed to chemicals, they can influence epigenetics, or the cues that turn genes on and off. These patterns can later influence how genes are passed on to offspring.

A man’s sperm can be affected by chemical exposure and the abnormalities can affect his children, for example. Chemicals may also affect stem cells, which the paper argues “pav[es] another way for effects that may last a lifetime.”

The chemicals the paper singles out are many of the usual suspects: bisphenol A (BPA), phthalates, dioxin, and some persistent organic pollutants. Many of these are known to be endocrine disruptors, or chemicals that interfere with the body’s own hormone systems.

Read More: Chemicals That Are Lurking in Cosmetics »

Smaller Exposure Over Longer Periods

Though the preconception exposure is the paper’s newest, most startling finding, Grandjean emphasized that chemical exposure also continues to occur during key vulnerable periods that have been identified in the decades since the photo of the Japanese mother was taken.

“In addition to the new mechanisms mentioned, many environmental chemicals are accumulated in the body over time, sometimes for several years,” he said in an email to Healthline. “This chemical burden may affect the conception, or it may affect the fetal development later on, as the mother will generally share her chemicals with her child — and that continues after childbirth, as she may also excrete these substances in the milk.”

For instance, developmental exposures to phthalates, persistent organic pollutants, and BPA increase a child’s risk of becoming obese.

Cancer risk is also tied to long-term exposure to endocrine disruptors. BPA interacts with the body’s progesterone pathway, contributing to cancer risk of progesterone prescribed after menopause.

Although the role of the same chemicals in prostate cancer had been a source of confusion, since the cancer responds to androgen and not estrogen, newer research suggests that endocrine disruptors might affect cancer stem cells.

Read More: Exposure to BPA During Pregnancy May Raise Diabetes Risk »

Push for New Ways to Regulate Chemicals

The paper does not shy away from the political implications of this line of thinking.

The United States should change the way it regulates chemicals, the authors argue. Safety testing should look not just at massive, short-term exposure but also low-level exposure over a lifetime.

Testing models should also consider how various chemicals might interact to be more dangerous than the sum of the parts, it says.

Maffini, who specializes in chemicals in food, agreed.

“There are food additives that were approved in ’60s or ’70s, and they’ve never been reviewed. You look at this paper and you see how much the science has advanced since then,” she told Healthline. “We can’t continue to test like we were doing in the 1950s. Testing has to be related to current scientific knowledge.”

“There’s strong agreement within the academic scientific community that the current system needs to change,” Maffini said.