This actually breaks down many of the sources of anxiety during pregnancy: The ultrasound is equivalent to X weeks, which is the gestational age estimated backward from the BPD/HC/AC/FL against a reference table; the EDD gestational age is the calendar gestational age. A difference of a few days between the two is, in most cases, just normal variation around the mean for the reference population—not that the baby is lagging behind by a few days. A more reliable way to ask is: For this ultrasound, what percentile does the AC and EFW fall into for that calendar gestational age, and compared to the last one, has the slope dropped a percentile rank?