how accurate is a dating scan after 17 weeks?

[emmalou28]

hi, i was wondering if anyone can help me, i found out i was pregnant at 17weeks, which was determined through an ultrasound, for my EDD. I then had another scan at 28 weeks where all the measurements were taken, however it was measuring for around 31weeks rather than 28. Could the original scan (EDD) be a few weeks out or is it just a large baby? i know they become less accurate after 13weeks but wasn't sure of the margin of error. any help would be much appreciated! thanks em x

My understanding, which may well be wrong, was that early scans are very accurate as they can use stage of development as an indicator - once you get past about 13 weeks pretty well everything is already there, so then you're left with average size. I'm not too sure how big the margins of error are.

You might be able to get an inkling, though, by looking at growth curves for babies (I've just found a "normal baby boy" weight and length curve, which shows length at birth has a range of between 46 and 54cm, with a mean of 50 and that "average" baby woulnd't reach 54cm till at least a month - so you could say that length at term is only an indicator of due date +/- a month).

Hope that helps and makes some kind of sense!