First trimester structural ultrasound?

So I have a 2d/3D/4d ultrasound at 13w3d for this "First trimester structural ultrasound" which I don't understand. Wouldn't it be done on the second trimester already? Has anyone had this ultrasound done, if so at how many weeks? I know about a nuchal translucency ultrasound done at 12 weeks but never past that... So I am a little confused. I need some input. Thanks!