Looking for a bit of advice to see if any of you with twins have had a similar situation....

On Wednesday I went in for my first scan and was shocked to discover I was carrying twins!! My cycles leading up to me falling pregnant were long (around 42 days) so I knew it was unlikely I would be the full 12 weeks. I actually thought I would be more like 10 weeks but the sonographer said I was measuring 11 weeks which I was pleased about. Usually at this scan they would do the screening tests for Downs Syndrome etc however he said that because I was only 11 weeks they were not quite big enough and I have been booked to go back in on Monday for the screening tests and a further scan to give me a more accurate due date. Does this seem normal? I’m starting to feel a bit worried because I’ve read everywhere that these screening tests are done between 10-13 weeks so can’t understand why they couldn’t be done if I was measuring 11w? Is it different with twins?