Urban trees and the risk of poor birth outcomes

Abstract

This paper investigated whether greater tree-canopy cover is associated with reduced risk of poor birth outcomes in Portland, Oregon. Residential addresses were geocoded and linked to classified-aerial imagery to calculate tree-canopy cover in 50, 100, and 200 m buffers around each home in our sample (n=5696). Detailed data on maternal characteristics and additional neighborhood variables were obtained from birth certificates 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). Results suggest that the natural environment may affect pregnancy outcomes and should be evaluated in future research.

Introduction

There is increasing evidence that greenness can improve the health of urban residents. The pioneering work in this field was done by Ulrich (1984), who showed that patients recovering from gall-bladder 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 fields including evolutionary biology and psychology. Research has concluded that the natural environment, in general (Frumkin, 2001, Wilson, 1984), and trees specifically (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. Specifically, 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 modified 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.