Send Auto Email in Bug_AfterPost event

I have written a code in Bug_AfterPost event to Send Auto Email when new defect is submmitted.

Sub Bug_AfterPost
On Error Resume Next

if Bug_Fields.Field("BG_Status").Value = "New" Then

' Send email

end if

I am able to send auto emails when new defect is raised. However email is also sent if Assigned to field is changed later and cursor moved to other row.

I want emails to go only when new defect is submitted. How can i restrict this event not to execute if any changes in existing defect.

Thanks