Yes! I am measuring small.
I am another one who is tall and slim-ish - I am 5'11" and my pre preg bmi was around 19.
I am a whole 4 weeks/4cm out though (I am 28 weeks, measuring 24) and they do seem to be quite worried....
Actually, its a bit horrid as I measured small at my 28 week mw appointment on Monday and she immediately called the hospital for an "emergency" appointment (you can imagine my horror at this point as I am lying on the bed thing with my dress round my neck!!), so now I am waiting to see the consultant and have a scan on Thurs morning, tomorrow. Its a bit of a nasty waiting game.... the mw thinks its my build coupled with the fact that she thinks the baby is transverse, but needs to be sure. Baby's heartbeat was good though and it is kicking, so she was content not to send me in that day.
In the meantime, I have been told to obsessively very carefully monitor movements in advance of the scan. If the baby is moving plenty, apparently it can't be too unhappy in there, so hopefully it won't be low amniotic fluid and IUGR because of comprimised placenta or whatever.
I did feel better after talking to my Mum who said she measured small (but not by as much) throughout all her pregnancies... she is 6' and her pre preg bmi was 17!!! (ex-model... grr!). We were all on the small side (none of us exceeded 6lbs 10oz, even at term), but fine!
PS - interesting that most of you are expecting girls... I chose not to find out at my 20 wek scan but have always been pretty convinced its a girl!
Lets keep eachother posted as to how we get on with scans etc... I hate worrying alone. Its scary!