I seldom read this board, but just happened upon this...and I tell you this only because I do wish I had known during this last pregnancy.
I measured really small in the third trimester, week after week. My first baby was only 5lb7oz, so I didn't worry, and mostly, neither did my midwives. We figured the babe was just another pipsqueak like my first. Then, I went to 41 weeks and had an ultrasound to check fluid, and the baby was TEENY. Really teeny. Under 5 lbs, and with a placenta that was barely letting anything through. They said she had stopped growing at 34 weeks. I was sure of my dates. We went off to L&D and tried induction, but the baby was not strong enough (tons of decels), and I ended up with a c-section. She was 4lb11oz -- healthy but so small for a post-date babe. After a week of hypoglycemia she was fine, and at 3 months, she's already about 11 pounds, so all's well that ends well, but still...
If I had known all along that this was happening, I think I'd have tried an induction earlier, to do it more gently (this was one walloping dose of pitocin, an effort to get her out quick) over more time. It may be worth doing an ultrasound to check...obviously that's between you and your midwives/doctors, but I wish I'd done it.
I hope the measurements are wrong for you, truly I do. Good luck!