Which scan considered more accurate for dating?

[blessedatlast]

I think the 12wk scan is the most accurate.. I knew my dates because I was on clomid but measured 5 days behind what my dates were.. (which was impossible because I got a BFP at 11dpo, by their dates I would have got my BFP at 6dpo IYKWIM) At my 12wk scan though I was right on dates and have been measuring bigger ever since. I personally think the 1st scan is too small to measure size accurately..

ETA:

Based on this information, which should I take as my EDD? Which will the birth centre take as more accurate, the earliest scan or the one showing viability with a heartbeat?

To actually answer this question, I think they will take the 1st scan as EDD, mainly because it is the closest to your LMP as well, but not sure. If you know your cycle you work out your due date and tell them, I did, but somehow according to the hospital my EDD is neither my scan, LMP or my date Lol.. They just picked one. (it was only 1 day off my dates though)

[Ashlee]

With my DS, I had a scan at 5 weeks 4 days and they used that as my EDD. At my 12 week scan I was measuring 4 days ahead, but the GP and the hospital made me keep my earliest scan date as my EDD, hence why I'm conflicted!!!

[kass]

My scans have consistently been three days behind where they should be. I've been through ivf, so I know exactly when o and fertilisation occurred, but the hospital is going with the date according to the scans. i figure it's just an estimate anyway, plus if I go over (which family history suggests I will) I'll have an extra three days up my sleeve before they start pushing induction

[blessedatlast]

With my DS, I had a scan at 5 weeks 4 days and they used that as my EDD. At my 12 week scan I was measuring 4 days ahead, but the GP and the hospital made me keep my earliest scan date as my EDD, hence why I'm conflicted!!!

Totally understand. With the twins I had no idea of my dates, so just went off 1st dating scan which was close to my LMP date. At the 12wk scan I was 5 days ahead and remained that way in subsequent scans but they still went off 1st scan..

There can be a couple of factors influencing what you would prefer. I would prefer the hospital thinks I am further along because IF i did go early, it could be the difference between Bub being legally recognised as a birth at 20wks, or being legally recognised as viable at 24wks.. (Commonly people don't worry about this though, I think it's just because of my history) Then there's the other side of the coin as Kass mentions, if they think you are earlier, they will let you go longer before talk of intervention.. At the end of the day though EDD's don't mean much at all!

[Traveller]

The earlier a scan is done, the more accurate it is. As your first scan ties in with your LMP they are likely to use this one for giving you an estimated due date.

[Tuesday's Child]

They usually go by LMP unless the difference between in EDD from LMP and u/s is more than 10 days in which case they'll use the u/s. At least this is what was done by the OBs I worked on clinical trials with.