How inaccurate is it possible for dating scans to be?

[TheOnlySeven]

When I had my dating scan my EDD was put back by an entire fortnight, I honestly have no idea how my dates could have been so far out. Anyway my other 3 DCs all arrived between 35-36 weeks, I'm now 33+4 and definitely nesting, I've only nested once before but went into labour the following day. Just curious as to whether I could actually be further along than 33 weeks.

The sonographer told me they can be out by a week either side!

Ok thanks, could only be a few more days then.

If you are interested in some stats have a look through the charts here, these are the official values that nhs sonographers use to date your pregnancy: fetalanomaly.screening.nhs.uk/fetalanomalyresource/images/stories/Downloads/2.4.1_Base_menu/fetal_size_and_dating-charts_recommended_for_clinical_obstetric_use._journal_article_2009.pdf

Basically, it says that if your baby measured 54mm at the scan, you would be given a date of 12 weeks 0 days as that is the average size (50th centile) for that gestation. However babies between the 5th and 95th centile would really be distributed in a bell-shaped curve anywhere between 11+2 and 12+5 weeks, and of course, a few percent of babies would be even further outside those dates but still growing and developing perfectly normally!
So I think there is a reasonable margin of error. Good luck with the nesting and everything else!

Thanks, so I guess being a week out would be realistic but not a fortnight. Interesting to know though.

When I looked into this, a scan done between something like 11 & 13 weeks will have a 95% chance of being accurate within 3 days (they tested on people who had IVF so conception date was known precisely). That still leaves a 5% chance of > 3 days out.