Scale a rectangle from center

[belo]

hello,

i have a simple rectangle.
i would like to scale it to 200 but the scaling always begins from top-left corner of the rectangle.
how can i scale it from the center?
thank you!

rect.onPress = mousePress;
rect.onRelease = mouseRelease;
function mousePress():Void {
    var item:MovieClip = this;
    item._xscale = item._yscale = 200;
}
function mouseRelease():Void {
    var item:MovieClip = this;
    item._xscale = item._yscale = 100;
}