Life is stressful. Most of us have learned to live with it as a bad side effect of modernity. But stress has been with us for a long time — ever since the first sabre-toothed tiger chased the first human. And we’re just beginning to know the profound ways our stress changes not only our physiology, but also our future children’s.

Despite what we may feel, stress isn’t inherently bad. Some types of stress are good — a promotion with bigger responsibilities, the birth of a baby. Other types are tolerable, like the death of a loved one, or a financial loss. And then there is the kind of stress that is toxic, a trauma “so bad that we don’t have the personal resources or support systems to navigate it, something that could plunge us into mental or physical ill health and throw us for a loop,” writes neuroscientist Bruce McEwen for Aeon.

Stress can trigger inflammation

But that simplified explanation can mean stress is misinterpreted by the average person. All stress, collectively — good, tolerable and traumatic — past a certain point causes the critical biological responses that help keep us alive to become overactive, threatening to our ‘allostatic load,’ that is, our physiological equilibrium. McEwen explains:

Allostatic overload can be the result. And it plays havoc with the body’s systems, leaving them operating in subpar ways that, in the long term, contribute to high cholesterol, diabetes, cardiovascular disease, obesity, depression, inflammation and immune suppression. On top of that, toxic stress alone is known to change our brain structure, to limit the brain’s ability to adapt and grow and to increase the size of neurons in brain regions responsible for anxiety and aggression, among other things.

Little wonder then, that studies are now showing stress changes parents’ biological functions in ways that impact their children before those children are even born — with potentially long-term effects on the next generation.

Maternal inflammation during pregnancy can affect kids’ development

A 2007 meta-analysis maternal stress during pregnancy could change the in-utero environment for the fetus, and increased later chances of emotional or cognitive development problems in children, including language delay, anxiety and ADHD. Further, the researchers estimated maternal stress during pregnancy was responsible for roughly 15% of emotional and behavioral problems in children. But having established the link, the study provided no insight into how.

One possible explanation is that stress can trigger inflammation in the body, which in turn triggers a response by the immune system to reduce it. A study, only just published in the Journal of Neuroscience, suggests that it’s this immune response in mothers, when it occurs in the third trimester of gestation, that affects long-term brain development in kids. A team lead by Dr Bradley Peterson, director of the Institute for the Developing Mind in the Department of Pediatrics at Children’s Hospital Los Angeles, tracked a group of pregnant women known to be at high risk of psychosocial stress and resulting inflammation.

Researchers drew blood from mothers-to-be during their third trimester and tested levels of IL-6 and CRP — two proteins that are found at higher levels when the immune system is activated. They also monitored fetal heart rate as an indicator for nervous system development. The team found that CRP did correlate with variability of the fetal heart rate, which is influenced heavily by the nervous system, indicating that maternal inflammation was already beginning to shape brain development.

Brain imaging at birth revealed a striking finding — significant changes in the communication between specific brain regions in babies whose mothers had higher IL-6 and CRP levels. These brain regions are known collectively as the salience network, whose job is to filter stimuli coming into the brain and determine which deserve attention.

“Our brain is constantly receiving information from our bodies and the external world,” Peterson explains. “The salience network sifts through that information and decides what is important and warrants action.”

Further, the team found significant changes in the scores of toddlers born to mothers with elevated levels of both IL-6 and CRP when the children were tested for motor skills, language development, and behavior at 14 months, suggesting maternal stress during pregnancy may affect kids’ brain development long term.

Paternal stress before birth can, too

Also becoming clear is the long-term effect a father’s stress has on the brain development of his children. Brand new research by University of Maryland neuroscientist Tracy Bale has shown paternal stress can change male sperm, affecting genetic material known as microRNA on the most fundamental life-shaping levels; microRNA plays a key role in epigenetics, that is, which and how genes become functional. It’s a process at the juncture of Nature versus Nurture, one that “drives the seamless integration of experiences, both good and bad, acting on our genetic code over our life course,” McEwen explains.

Bale’s findings suggest that even mild environmental challenges, the kind, perhaps, that McEwen references, can have a significant impact not only on us, but also on the development and potentially the health of future offspring — compounded, of course, by the stress they experience in their own lives. Indeed, research is starting to make clear the deep impact sustained childhood stress has on kids’ development.

As significant as these studies are, they’re only the beginning of a long road to understand the long-term effects of parental stress on kids.

“This finding fills in a missing piece,” says Peterson — but only one of many.

Until scientists find the answers, McEwen recommends living a generally healthy lifestyle that includes mindfulness practices, like meditation, and daily physical exercise, which can mitigate stress’s effects on the brain. We can’t avoid stress, but we can shoot for a lifestyle that mimics and reinforces the physiological equilibrium our bodies need.