Can contact with nature lower the preterm birth risk?

The World Health Organization estimates that approximately 15 million preterm infants (20-36 weeks) are born worldwide each year. In 2014, China had the second-largest number of preterm births (PTBs), behind India. Among children below the age of 5, the leading cause of mortality is complications from PTB, leading to nearly 1 million deaths globally in 2015. The incidence of PTB has been increasing over the past 30 years in China due to assisted reproductive technology and the two-child policy launched in 2015. PTB is the leading risk factor for infectious or chronic diseases in early childhood and further life. Finding effective interventions to reduce PTB risk could benefit public health.

Numerous studies indicate that exposure to greenness (spending time in an environment with dense vegetation) could be beneficial for health in the case of cardiovascular disease, mental health, and birth weight. Although those conclusions were inconsistent, few studies reported the protective effect of greenness exposure on birth weight. Plenty of the studies found no associations between greenness and PTB, and a few even indicated an inverse association.

Potential mechanisms of the positive effect of greenness exposure include supporting mothers' physiological or psychological health and increasing the possibility of physical activity and social interaction. A green environment also decreases exposure to air pollutants. Due to inconsistent conclusions between greenness and PTB and the significant burden of the condition in China, the research team led by L.Zhang decided to perform a thorough investigation.

The impact of exposure to greenness was assessed on a cohort of 3,751,672 singleton births obtained from a national birth cohort in mainland China. A satellite-based Normalized Difference Vegetation Index (NDVI) was used to estimate the greenness. NDVI ranges from -1 to 1, where high positive values represent a high-density vegetation cover while 0 or negative values represent bare land or water bodies. PTB was subcategorized into extreme PTB (EPTB, 20-27 weeks), very PTB (VPTB, 28-31 weeks) and moderate-to-late PTB (MPTB, 32-36 weeks). Gestational age (GA), which is a common measure of the age of the pregnancy, was investigated as well.

The research proved the inverse associations between greenness and PTB and positive associations between greenness and GA. An increase of 0.1 NDVI exposure throughout the whole pregnancy was visibly associated with decreases in PTB, EPTB, VPTB, MPTB, and an increase in GA. What is more, mothers of lower socioeconomic status and older than 30 years or younger than 24 years were observed to benefit more from exposure to greenness.  Results of the study suggest the potential protective effects of greenness such as on PTB in China and might indicate an innovative and cost-effective strategy to reduce PTB risk, especially in developing countries. An example could be shinrin-yoku - a Japanese technique of making contact with the forest.

Source: Zhang, L., et al. (2022). "Effects of greenness on preterm birth: A national longitudinal study of 3.7 million singleton births." The Innovation: 100241.

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