Im not sure...

I had one at 7w 5d coz i had a small bleed, and i was so so sick and dehydrated I had to stay in hospital, so they did one just to check if bub was okay, thats when they dated it.

Then at 12weeks

And again at 19w 3d (I was leaving for a holiday in England so they did it early)

And then I had a 3D/4D one at 31weeks....but that was just for enjoyment

Congratz btw