As you know, i has this with Lily which they found at the 19 weeks scan. I then had to go to a specalist for another scan at 21 weeks to check the likely hood of downs which came back 1 in 3300 or something like that. The doctors never even suggested a growth scan and she came out a healthy 9lb 7oz. So I wouldn't be too worried. Apparently it's rare that something bad will go wrong.