I had three different doctors (including a specialist) insist that my son hadn't grown at all during the last three months of my pregnancy. Ultrasounds "confirmed" that he would be a "micro-preemie". I was scheduled for c-sections and inductions (all which I didn't show up for). He came out shortly after all that chaos at almost 9 pounds. :-/ So the measuring small thing really doesn't mean much to me, especially if your pregnancy has been otherwise healthy. And if your midwife was really concerned, you would know it asap. She would definitely be letting you know. In the case with my son, he kept flipping from breech to head down, so that's why his measurements were so funky. Not sure how they managed to goof up so many ultrasounds though.
I bet you and the baby are fine. The measurements are a good thing to pay attention to, but they are by no means a perfect science.