New moms who live on tree-lined streets may be somewhat less vulnerable to postpartum depression, according to a new study — the latest to link "green space" to better mental health.
The study, of medical records from more than 415,000 new mothers, found that those living in urban areas with more tree coverage had a lower risk of being diagnosed with postpartum depression, versus women from less-green neighborhoods.
The link was not explained by factors like household income, or mothers' race or education level.
Experts said the findings do not prove that living among trees lowers the likelihood of postpartum depression. But they do add to a body of research suggesting that having green space within sight is a boon for people's mental well-being.
The study also points to one reason: physical activity. It's a lot easier to go out for a walk when you live in a tree-lined neighborhood, with its built-in shade and better air quality.
And for new mothers, that may be especially important, said senior researcher Jun Wu, a professor at the University of California, Irvine.
They have little time and are likely exhausted, Wu said, and getting to the park, especially if they have to drive, might be out of the question. Stepping outside into the fresh air and shade is much more doable, she suggested.
Kathleen Wolf is a research social scientist at the University of Washington's College of the Environment, in Seattle.
Wolf said that studies done across cultures have shown the relationship between exposure to green space and better mental well-being.
And while the new study focused on the role for exercise, there are many other reasons that being among trees can be a balm for the mind, both Wolf and Wu said.
For one, it can help people de-stress. Research has found that, on average, people who regularly spend time in green spaces have lower blood pressure, heart rate and levels of the "stress" hormone cortisol.
And when you're walking around the neighborhood, both researchers said, there's a chance for social connection — simply saying hi to a neighbor or petting someone's dog, for instance.
Wolf said that time outdoors can also help us broaden our perspective and put life's daily stresses into context.
"People have a tendency to ruminate on the bad things," she said. "Time in nature can help lift us out of that rumination."
The findings — published online March 6 in The Lancet Regional Health — are based on records from more than 415,000 women who gave birth at Kaiser Permanente Southern California medical centers between 2008 and 2018.
Wu's team used various sources, including street-view images and satellite data, to estimate the amount of green space around each family's residence.
Overall, around 10% of new moms in the study were diagnosed with postpartum depression. But the risk dipped when they lived among more greenery: For every 10% increase in "street-level" green space, the risk of postpartum depression declined by about 4%, the investigators found.
That was after the researchers weighed other factors, like the typical neighborhood income, women's age, race and education level, and whether they had any pregnancy-related health conditions.
Trees, specifically, stood out as protective, while grass didn't seem to cut it. Nor did having a nearby park.
Because Wu's team had information on the women's physical activity habits — self-reported in their medical records — they looked at whether exercise explained part of the findings. And it did account for a small portion of the link between green space and lower depression risk.
Both researchers said that this study, along with previous ones, has implications for community and city planners.
For one, they said, it's known that there are disparities in access to green space in cities, with many of the parks and other urban oases concentrated in higher-income neighborhoods.
"For a long time," Wolf said, "the attitude was that parks and trees are a nice thing to have."
But now, she said, there is plenty of evidence that green space is also a matter of public health. And Wolf said she does think the issue is gaining more attention.
"I think the research really is changing how city planners and managers are looking at green space," she said. "Nature is good for us, and it should be a public good."
The U.S. National Park Service has more on nature's health benefits.
SOURCES: Jun Wu, PhD, professor, environmental and occupational health, University of California, Irvine; Kathleen Wolf, PhD, research social scientist, University of Washington, College of the Environment, Seattle; The Lancet Regional Health, March 6, 2023, online