Residential greenness exposure and preterm birth and small for gestational age: potential association, interaction with fine particle matter and mediation role of systemic immune inflammation.

Residential greenness, as an essential aspect of the environment, contributes significantly to alleviating the negative impacts of accelerated urbanization and promoting sustainable urban development. However, the associations between residential greenness exposure and the risks of preterm birth (PTB) and small for gestational age (SGA), as well as the potential biological mechanisms underlying these associations, remain underexplored. In this study, we assessed the associations of residential greenness exposure across different periods of pregnancy (first, second, and third trimesters, as well as the entire pregnancy) with the risks of PTB and SGA, examined the interaction effects between greenness and PM2.5, and quantified the mediating role of the systemic immune-inflammation index (SII) in the associations between greenness and PTB or SGA using mediation analyses. We found that each 0.1-unit increment within 500-meter buffer (NDVI-500m) throughout pregnancy was associated with a 5.0-6.0 % lower risk of PTB and a 4.0-10.0 % lower risk of SGA. The third trimester emerged as the period during which greenness exposure had the most pronounced impact on these adverse outcomes. Non-linear exposure-response relationships were observed between residential greenness and the risks of PTB and SGA. Significant interactions were observed between NDVI-500m and PM2.5, whereby greater greenness combined with lower PM2.5 concentrations was jointly associated with the lowest risks of PTB and SGA. The SII mediated 8.6-16.1 % and 4.2-12.1 % of the effects of NDVI-500m on PTB and SGA, respectively. These findings support that exposure to residential greenness lowers the risks of PTB and SGA, and mitigates the adverse impacts of PM2.5 on these outcomes, and may partially exert its effects through SII.