it is a bit early... Bubs is still growing, and they normally do this next scan at around 18-20? weeks from memory. So at 17.5 weeks, putting you back a week is probably the right thing to do

Remember, at this stage they are growing around 1cm a week, and lots of things are still forming. Could well be that some things simply aren't ready to be seen yet...

Good luck for the scan next week