Measurments aren't always correct, but more so rough estimates. Buy, average weight is about 8 lbs. So don't worry :) they might take the growing easy now haha, but who knows, maybe they'll also come a couple weeks early. And two pounds in month could happen, but it'll still be a normal weight :)
They thought my son was gonna weigh a lil less than 9 pds but when he came out he only weighed 7'7