Doctors calculate pregnancy using a 40-week calendar, though from the date
of conception, pregnancy
is only 38 weeks long. They go back and start the count on the first day of your last period. 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.