Dr im 16.6 weeks preg on today's ultrasound all measurements are accurate(17.2weeks showing) but fl is 16.0 weeks, why it like this need of worry?:(

Numbers: The original calibrating standards for U/S measurements on fetal growth were based on a population of Scottish women who had a history of very regular menses. All data sets have variability in them. Think of polls which are published and they always (or should say) answer is blah plus or minus 3 percent. Any quantitative test has to be understood as an approximation to the "true" answer.