The association between tree planting and birth outcomes

Multiple studies have found that exposure to greenspace during pregnancy is associated with increased birthweight (Hu et al., 2021) and a reduced probability of a small-for-gestational-age birth. However, because exposure to greenspace is associated with demographic correlates of health, studies may suffer from residual confounding. In addition, results may be influenced by the self-selection of healthier women into greener neighborhoods. We address these limitations by using the number of trees planted around maternal address as a novel exposure metric. Compared to existing tree cover, new tree planting is less prone to residual confounding or self-selection bias. We found that each tree planted within 100 m of maternal address within the last 10 years was associated with a 2.3 g (95 % CI: 1.5-3.0) increase in birth weight. In addition, we found that 1-SD increase in tree planting was associated with a 4.3 % (95 % CI: 0.3 to 8.2 %) reduced risk of a small-for-gestational-age birth and a 4.9 % (95 % CI: 0.3 to 9.3 %) reduced risk of a preterm birth. All models controlled for mother's race/ethnicity, education, major road density, and existing tree canopy. Our finding that both newly-planted and established trees are associated with improved birth outcomes strengthens the evidence that trees may be an effective public-health intervention.