The software has certain measurement scales based on data from large populations, and your baby’s measurements are put into this scale.
In other words, by comparing your baby’s measurements to the data from this large collection of measurements, the ultrasound can then tell how far along your baby is.
The simple rule in ultrasound is that when the due date based on ultrasound doesn’t vary from the mother’s dates by more than a week, stick with the mother’s dates; if the ultrasound disagrees by more than a week to ten days, it becomes wiser to rely on the ultrasound.
This doesn’t apply later on in pregnancy for the reasons above. If two ultrasounds one month apart determine coinciding due dates, especially if they agree with the mother’s date based on a last period, the accuracy can be within a couple of days. Babies have their own clock and can come anywhere from three weeks before this exquisitely determined due date till two weeks after.
The femur (thigh bone) length seems to hold on to it’s accuracy longer than the other parameters, but after 36 weeks it isn’t foolproof.