The National Physicians Cooperative. The National Physicians Cooperative is a federation of fertility programs that work together to share practice plans and ensure that patients receive the most authoritative and timely care. There are four core centers in the USA where research is being conducted (denoted in color): Northwestern University, Chicago, IL; University of Pennsylvania, Philadelphia, PA; Oregon National Primate Research Center, Beaverton, OR; University of California, San Diego, CA. An additional 68 allied centers are located across the USA (denoted by black dots; numbers indicate more than one center in the same location). A national fertility helpline (Fertline) is operated at Northwestern University on behalf of the entire network.