If stress can cause pre term birth, how does it happen?what exactly triggers it?

Stress: A Marker: No one really knows for sure whether or not stress really causes preterm birth. Stress can be defined in so many ways and people handle stress differently. Stress can be a function of your environment which may also cause ptb. For example, someone poor who lives in a high crime area experiences more "daily stress" but this population may have more infections/malnutrition which may also cause ptb.