Connector assemblies that are separate from medical lead extensions provide features such as bores for receiving both a medical lead and a medical lead extension and provide electrical connections between connectors of the leads and connectors of the lead extensions. Connector assemblies may include additional features such as contours and wings that reduce subcutaneous erosion. Connector assemblies may also include retention structures such as movable clips that are moved into engagement with leads and lead extensions to retain them within the connector assembly. Integrated lead extension connectors may also include contours and wings as well as retention structures including movable clips.