Pregnant is a synonym of Significant. Significant is a synonym of Massive. Massive is a synonym of Substantial. Substantial is a synonym of Real. 
Sustainable is derived from Sustain. Sustain is a synonym of Hold. Hold is a type of Cover. 
Pregnant <--Synonym--> Significant <--Synonym--> Massive <--Synonym--> Substantial <--Synonym--> Real 
Sustainable --DerivedFrom--> Sustain <--Synonym--> Hold --IsA--> Cover 