You've heard all the obvious benefits of urban trees -- shading buildings, sheltering wildlife, filtering air pollution, stopping erosion. A new Portland study suggests a more surprising benefit: healthier newborns.

Researchers used satellite images to compare tree cover around the houses of 5,696 women who gave birth in Portland in 2006 and 2007. Pregnant women living in houses graced by more trees were significantly less likely to deliver undersized babies.

Tree cover made no difference in the rate of pre-term births, but researchers found a consistent link to the prevalence of infants who were small for their gestational age. For each 10 percent increase in tree coverage within about 50 yards of a home, the rate of undersized newborns decreased by 1.42 per 1000 births. As it stands, about 70 of every 1,000 newborns in Portland are small for gestational age.

"Maybe it sounds a bit daft at first," says lead author

, a scientist with the U.S. Forest Service's Pacific Northwest Research Station in Portland. But he says it's plausible that having lots of trees nearby counteracts the stress experienced by pregnant women.

Studies in animals and people make clear that maternal stress is harmful to a developing fetus and can increase the probability of underweight birth. In a variety of human clinical trials, exposure to nature and greenery significantly reduced people's stress levels and helped them withstand high-stress situations.

"That may be the mechanism," says Donovan, a specialist in forest economics whose work for the Forest Service includes studying urban trees and their effects on crime, energy use and health. The birth study, co-written by researchers from the Multnomah County Health Department, Drexel University and the National Institute of Standards and Technology, was

by the journal Health & Place.

Dr. Stephen Fortmann, a senior investigator at the Kaiser Permanente Center for Health Research in Portland who was not involved in the study, finds the results intriguing. "It points out that some of the neighborhood level factors that effect health might work in ways we haven't thought about," Fortmann says.

But he says the results have yet to be replicated and remain far from conclusive. "The issue with any observational epidemiological study is confounding. Is there a causal relationship here, or is something else going on?" he says. "In this case, one immediately thinks there is probably something else going on –neighborhoods with a lot of tree cover, even in Portland, are very different from those with little tree cover."

Women located on leafier Portland streets were more likely to be younger, white and non-Hispanic, have fewer previous births, and live in newer and more expensive houses. To test for the impact of tree cover on birth outcomes, Donovan and co-authors used a statistical model to subtract the known effect of the mother's age, ethnic background, household income, education, use of prenatal care and many other variables that can influence fetal development.

"This isn't the final word," Donovan says. "Our point is, look, here are some interesting results, let's look at this some more." For now, the study suggests that Portland could look forward to three fewer undersized newborns per 1,000 births if the city were to achieve

of boosting the area covered by trees to 33 percent.

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