The PrevSegment property returns the previous segment that is adjacent to a node, or it returns Nothing if no such segment exists.
Examples:
The following VBA example creates an ellipse on the active layer, converts it to curves, and returns its third segment.
Sub Test()
Dim s1 As Shape
Dim sg As Segment
Set s1 = ActiveLayer.CreateEllipse(0, 0, 2, 2)
s1.ConvertToCurves
Set sg = s1.Curve.Nodes(s1.Curve.Nodes.Count).PrevSegment
End Sub