By Geoffrey H. Donovan, Yvonne L. Michael, et al.
Portland, OR (November 7, 2010)- This paper investigated whether greater tree-canopy cover is associated with reduced risk of poor birth outcomes in Portland, Oregon. Results suggest that the natural environment may affect pregnancy outcomes and should be evaluated in future research.
Residential addresses were geocoded and linked to classiï¬ed-aerial imagery to calculate tree-canopy cover in 50, 100, and 200 m buffers around each home in our sample. Detailed data on maternal characteristics and additional neighborhood variables were obtained from birth certiï¬cates and tax records. We found that a 10% increase in tree-canopy cover within 50 m of a house reduced the number of small for gestational age births by 1.42 per 1000 births (95% CI-0.11-2.72).
There is increasing evidence that greenness can improve the health of urban residents. The pioneering work in this ï¬eld was done by Ulrich (1984), who showed that patients recovering from gallbladder removal surgery in a room with a view of a natural scene were discharged quicker and required less pain medication than those who recovered in a room with a view of a brick wall. More recently, observational studies have shown that greenness is associated with lower obesity (Bell et al., 2008), perceived general health (Maas et al., 2006), morbidity (Maas et al., 2009b), and mortality (Mitchell and Popham, 2007). The relationship between health and the natural environment has been studied in other ï¬elds including evolutionary biology and psychology. Research has concluded that the natural environment, in general (Frumkin, 2001; Wilson, 1984), and trees speciï¬cally (Perlman, 1994) can improve human well being. There has been no research, however, on the effect of greenness on reproductive health.
Past research has shown that birth outcomes are related to stress (Miranda et al., 2009), neighborhood-level economic deprivation (Messer et al., 2008; O’Campo et al., 2008), and social capital (Buka et al., 2003). Although these studies did not consider greenness, they suggest potential mechanisms linking greenness and birth outcomes. We address this gap in the literature by quantifying the effect of urban trees on adverse birth outcomes. Speciï¬cally, we tested the hypothesis that greater access to urban trees would reduce the incidence of preterm birth (PTB) and small for gestational age (SGA), both of which are major causes of neonatal and infant mortality as well as contributing to health problems in later life (Hack et al., 1995).
We chose to study the effect of trees on birth outcomes, because urban trees are an important element of the natural environment that can be more readily modiï¬ed than other natural amenities. For example it is easier to plant trees in a neighborhood than increase the size of parks or other open space.
The study sample consisted of all singleton live births in Portland, Oregon, during 2006 and 2007, where the mother’s address was a single-family home (n = 5696). Of these, 348 births were preterm and 397 were SGA (33 births exhibited both). Our analysis was conï¬ned to single-family homes because of practical
difï¬culties measuring trees around multi-family homes. We geo-coded houses by matching a mother’s address on a birth certiï¬cate to an address in the Regional Land Information System (RLIS) database, which contains coordinates for the centroid of each house’s lot. The RLIS database is maintained by Metro, which is the metropolitan Portland regional government with responsibility for urban planning and transportation.
Greater tree-canopy cover within 50 m of a house, and proximity to a private open space, were associated with a reduced risk of SGA. Results do not provide direct insight into how urban trees may improve birth outcomes. However, stress reduction is a plausible biological mechanism linking trees to SGA, as previous research has shown that maternal stress can increase the probability of underweight birth (Miranda et al., 2009), and exposure to natural environments can reduce stress (Ulrich et al., 1991). In addition, green space may act as a buffer against the negative health impact of stressful life events (van den Berg et al., 2010).
Improved social contacts are another possible psychosocial mechanism, as perceived levels of neighborhood social support are positively associated with infant birth weight (Buka et al., 2003) and the availability of large green spaces is associated with perceived social support (Maas et al., 2009a). Neighborhood greenness is also associated with greater levels of physical activity (Townshend and
Lake, 2009), and greater levels of physical activity during pregnancy may protect against SGA births (Gollenberg et al., in press). However, exercise is unlikely to be the sole mechanism whereby trees affect birth outcomes, as increased tree-canopy cover within 50 m is a localized effect, and one would expect most exercise in a neighborhood to take place further than 50 m from a house.
Although no observational study can prove a causal relationship, consider the following strengths of the study. First, it builds on past experimental work demonstrating that trees can improve health outcomes (Ulrich, 1984). Second, if trees were merely proxies for positive neighborhood characteristics, one would
expect that trees further than 50 m from a house would also be correlated with better birth outcomes, but they were not. Third, a wide range of individual and neighborhood characteristics, including many markers for socioeconomic status, were controlled for. Fourth, validation testing showed that results were not due to spurious correlation.
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