This is the way doctors have dated pregnancy for a long time, certainly from before the invention of ultrasounds, because the first day of a period is a noticeable signal
and known
date,
and ovulation is hidden
and happens on a
date not usually known. Of course a woman is not pregnant the first day she is bleeding from her period, but it is easy to calculate 40 weeks beginning then, knowing that the first week or two are simply an add-on for easy figuring.