The findings held up even when results were adjusted for factors such as neighbourhood income, exposure to air pollution, noise and neighbourhood walk ability.

"The research really suggests that greenness affects birth outcomes in other ways, such as psychologically or socially," said lead author of the study Perry Hystad, an assistant professor at Oregon State University in US.

In the study of more than 64,000 births, researchers found that very pre-term births were 20 percent lower and moderate pre-term births were 13 percent lower for infants whose mothers lived in greener neighbourhoods.

They also found that fewer infants from greener neighbourhoods were considered small for their gestational age.

Babies from the greener neighbourhoods weighed 45 grams more at birth than infants from less green neighbourhoods, Hystad said.

Babies born early or underweight often have more health and developmental problems, not just at birth but also as they continue to grow up, and the cost to care for pre-term and underweight infants also can be much higher.

The study appeared in the journal Environmental Health Perspectives.

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