Premature births are 30% higher in cities because of pollution
The article giving the details is at the above link please read that first.
As so many of my comments to the Daily Mail get moderated off I've copied my reply here.
But it may appear at the DM. This just in case it doesn't.

Atmospheric pollution from urban living also stops UVB reaching ground level so reduces the potential for vitamin D creation.

Another reason why pollution affects us all is that it acts as a stressor which increases cortisol production, high cortisol levels lower vitamin d status. Decreased serum vitamin D levels in children with asthma are associated with increased corticosteroid use." Cortisol is a corticosteroid.

Higher vitamin d status has been associated with fewer pregnancy complications "Adequate vitamin D during pregnancy reduces the risk of premature birth by reducing placental colonization by bacterial vaginosis species."free full text online.