Is Civil 3D 2018 Backwards Compatible?

Traditionally, since AutoCAD version 2000, Autodesk has changed its DWG file format after three versions. Since 2013, however, Autodesk decided to keep the same DWG format (AC1027) for 5 versions, until they finally decided to give AutoCAD 2018 a new DWG format (AC1032).Essentially, this means that if you save a drawing file in 2018, it will not be compatible with previous versions.
Source-CADsoft Consoulting