Using Tween class to resize from centre of the image

HEy guys,
I have a rectangle on my stage called BG.

I want it to resize while remaining centre aligned, but when I use the following code:

import mx.transitions.Tween;
import mx.transitions.easing.*;

var bgwidth:Tween = new Tween(bg, “_width”, Bounce.easeOut, 0, Stage.width, 3, true);
var bgheight:Tween = new Tween(bg, “_height”, Bounce.easeOut, 0, Stage.height, 3, true);

The top left corner of the rectanlge is aligned to the cnetre of the stage and it resizes from that point.

I;ve made sure the registration point of my shape is set to its centre also.

Can anyone help?