For my 12 week scan it was abdominal and they measured the babies neck well fluid around and then took a blood test for downs! And at my 20 it was to check everything else like arms, legs, heart and everything...
Some hosps say come with a full bladder, others dont, mind didnt, but its always best to go with a full one just incase!!
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The level 2 ultrasound for down syndrome which is performed before 14 weeks is both an abdominal ultrasound and possibly an interuterine one as well. It just depends on what they want to do. Make sure you drink lots of water before you go in.
You can have blood tests done,this has to be done before 13 weeks,these checks for downs etc.at 20 weeks they take measurements etc.
Usually that's done at 20 week ultrasound but at 12 weeks you should be able to have it done external but it depends upon the drs preference