Of all of these uses, dating the pregnancy is the most common reason to use ultrasound, particularly when the expectant mother cannot remember the date of her last period (as in breast-feeding or irregular cycles).
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.
does anyone know how accurate the dating scans are - is there a possibility of them being a 1hole month out?
had scans at 6w5d and 12 w5d and 20w4d - all match up to the same date but i remeber another date the month before when we did the deed but my partner did not complete in me - is there any chance the scans cud be 4 weeks out?
Most babies, except in cases of early IUGR and deformities, grow about the same until 20 weeks or so.