尺寸变化时保持物体中心固定

I have a circle that expands using CSS keyframes, with fixed positioning. The trouble is, as the circle changes size, the centre of the circle moves (whilst the top left corner remains fixed). How do I ensure the centre is fixed during the animation? Is there a way to specify the "origin" of the div so that's it's not the top left?I have a circle that expands using CSS keyframe