Once I had a waistline
It nipped in so nicely
But now my lovely waistline
Doesn't "nip," precisely
Now it's sort of dome-ish
A convex curve, at best
"Why so thickish in the middle?"
I'm sure they all have guessed
Once I had a waistline
I could place a belt around it
But now my lovely waistline--
Could I belt it if I found it?
(From a CD of tunes about pregnancy to be included with all copies of "What to Expect." In my dreams, at least.)

What's the tune to?