Personally, I wouldn't feel the need to explain something that is so grown-up to a 7 year old. Maybe 12 years old? but not 7. My tendency (when topics like this come up or are asked about) would be to change the subject or divert his/her attention to something more age appropriate. The 7yo imagination is too vivid for this topic to be discussed without putting fear in the child. Also, birth/death are very abstract concepts for a child this age.

Just my opinion for what it's worth.