A study examined whether living in a war zone compared to living in a non-war zone increased the risk of having a pre-mature birth. Women who had given a pre-mature birth were recruited and matched to women who had given a normal birth. Exposure data were collected by asking all participants if they were living in a war zone during their pregnancy. The study found that among women living in a war zone while pregnant, 250 had a pre-mature delivery and 4900 had a normal delivery. Among women living in a non-war zone of the same country while pregnant, 110 had a pre-mature delivery and 5100 had a normal delivery. What is the most likely design of this study?