That is why
pregnancy, which takes 38 weeks
from conception to full-term
birth, is counted as being 40 weeks long by doctors, textbooks, nurses, ultrasounds, everything. It's a common language and well known in the medical world, and continues to be used nowaways, to keep everything calibrated with the textbooks and the way doctors are taught. This leads to the rather peculiar phenomenon that the first two weeks of the GA count, the woman isn't actually pregnant yet.