I don't know about the elastic, but it sounds like you are talking about gathering the material at the bottom edge of the back yoke, or where a back yoke would be. That wouldn't be a special stitch, unless you have a gathering foot on your machine. I may not be understanding your description, though.

Are you thinking of "darts"?
One type of dart is called "Princess Seaming".
See link below.

I don't know of elastic ever being used in darts or princess seams...

Here is a link that might be useful: What is a Dart

Wouldn't it be easier to take in the sides of the shirt?

Are you going to do this yourself?

If you're taking it to a tailor or seamstress, he or she should know how to alter your shirt. You shouldn't have to tell him.

Thanks for the replies, everyone. While doing laundry, I noticed my daughter's shirt had the type of seaming I was asking about.
http://i155.photobucket.com/albums/s287/daisymaeplease/GardenWeb/015_zps7713322e.jpg
http://i155.photobucket.com/albums/s287/daisymaeplease/GardenWeb/016_zpsd4a98f2f.jpg
http://i155.photobucket.com/albums/s287/daisymaeplease/GardenWeb/017_zps236f270c.jpg
Evaf555, you're right. The seamtress should know what she's doing. Although, I always find it helpful to be armed with your own knowledge whenever possible. I took the shirt in and she suggested taking up the shoulders. Fingers crossed. :)

Trying pictures again...

This post was edited by NATEBEAR on Sat, Mar 8, 14 at 20:00

That's just a regular straight stitch sewn while the elastic is stretched out.

"shirring"

It's just several rows of large stitches, pulled up to gather the material. Or slightly stretched elastic, sewn to the shirt then relaxed.

What the two before me said. And very easy to do.

Love the kitties. Don't see how 'taking up the shoulders' will alter in the bust area.