When a client device loads a resource, a request for a first content item for a first content item slot is transmitted to a content item selection system. The content item selection system uses a predictive model to determine a predicted content item slot based on a document object model position of the first content item slot and a URL of the resource or a publisher identifier. Parameters for the predicted content item slot are used to select a subsequent content item for the predicted content item slot. The first content item and the subsequent content item are transmitted to the client device responsive to the request. The subsequent content item includes metadata indicative of the parameters of the predicted content item slot to be matched to a subsequent content item slot of the resource.