Which scan considered more accurate for dating?

[Ashlee]

Hi all

I had my first scan done at 5 weeks 2 days (by LMP), and measured in at 5 weeks 1 day. My GP left my EDD as per my LMP.

I had another scan today which would put me at 7 weeks by LMP or 6 weeks 6 days by first scan date, but I came in measuring at 6 weeks 3 days.

We saw a heartbeat of 132bpm and CRL of 7.1mm. My gestational and yolk sacs were both measuring 7 weeks 4 days.

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?

TIA!

[krysalyss]

My impression is that the first one is the most accurate as it has the least amount of individual variation as far as size, while most factors influence the size of the baby at later dates. Maybe someone else will be able to give you a better answer.

[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.

[MissEm]

I'd say the second, but don't be too surprised if at the 12 week scan (if you have it) they change your due date and gestation again. I have found with both my pregnancies they went by the due date and gestation they got from the 12 week scan.

[Ashlee]

Oh dear so much conflicting info lol I guess I'll just have to wait and see what the GP says, and then the birth centre. I think they are generally more inclined to use the first scan date, I've always heard the earlier they are the more accurate. Plus, with my sacs (ha that made me giggle) being over a week "bigger" than the baby, I'd say the average is in the middle anyway?

I'm only obsessed with EDD because I had an early baby last time, and a few extra days could be the difference between a birth centre and hospital birth.

[blessedatlast]

Aaah, that makes sense. I did wonder why it was so important to you Lol.. Let's just hope baby stays in and you get your Birth centre. you're on the Goldy aren't you? Where are you planning to go? JF??

[Ashlee]

I wish I had PHI, I'm just going to the public birth centre at Southport, which I've heard is basically of the same standard anyway! First appointment is on 26 October after my NT scan and bloods, so I guess they will meld all scans and other info together and choose a date. For now, I guess I'll just leave it as is, no point stressing my already stressed little head out more than necessary!!

Are you on the GC too?