I just had that issue, I was 31 weeks measuring 27 weeks, went to my ultrasound, and my baby girl is perfectly fine, she is just very low so that's why I was measuring off! Try not to worry, i know its hard not to though!
My Dr said I measured about 10-12 weeks at 18 weeks. Ordered an anatomy scan and the baby looked to be about 19weeks which was right on track. Dr didn't tell me my measurement this last time, but ordered another growth/anatomy scan. As long as they don't seem overly concerned about, I don't see a reason I should be. It's still one of those annoying thoughts in the back of my head though.
I measured 29cm at 32 weeks. I have a growth scan on tuesday.
My dr never bothered measuring as he considered it pretty inaccurate, we just went by what it said in our copy of the ultrasound report however thats an guesstimate as well. Was your midwife worried, or did she order a growth scan? If not I wouldn't worry.