As has been said, a one off growth scan shows nothing and 10th centile at 30 weeks could just be a natural size IYSWIM.
However, dd had IUGR. Hers was evident at my 20 week scan where she measured on about the 3rd centile. At the time they weren't sure if it meant anything as both dh and I are small. Luckily they erred on the side of caution and kept scanning me. Round about 34 weeks or so her growth slowed a bit and they were able to perform specialist doppler scans which were able to trace the flow from the placenta through the placenta and then round dd. These were fine. Anyway, they kept scanning me. At 37 weeks I went in for a scan and they performed the doppler scans to find that there was a problem (shown by the fact that the flow was more concentrated round the brain which shows there was a problem as the body knows this is where it is needed most - quite remarkable). I was admitted to be induced and my blood pressure was up.
To cut a very long story short, induction didn't work and dd was born a few days later by c-sec, weighing 4 lbs 9oz.
Most importantly, she was absolutely fine. She went to SCBU for a couple of hours to raise her body temperature but then was back with me. She grew like mad after that and has had absolutely no problems.
I was told that they don't know what causes IUGR but its probably to do with pre-eclampsia and that although I had no blood pressure problems until 37 weeks I may have had pre-eclampsia all along and this was how it manifested itself (in dd's growth).
Try not to worry (easier said than done). They will keep the baby inside as long as it is growing but will deliver if growth stops.
BTW, a friend of mine had similar problems with her dd1. She has just had dd2 and was told all along she was having the same problems until she got to 36 weeks when they scanned her and said the baby would be about 7lbs. She was born last week weighing 6lbs 13 oz.
Hope that helps (sorry I've gone on so much). Good luck !