Do you belive scan dates or lmp dates??

[Dee2009]

Hi all, I'm 29+3 days pg and had diabetes bt yesterday and a growth scan. At the scan they told me baby was big for the dates so i prob did have diabetes (although had to wait till today for results), have got results back which are normal!! When i had orginal scan i always thought i was 1week and 2days ahead and they pulled me back!! Am also pretty sure when that we concieved on our first week of hols!! Now they wont have it that i could be further along than i am, they just keep saying that scan dates are very accurate!! I hve to have another scan in two weeks time to check the growth again!!
Any advice would be great as not sure what to think.

The 12 week scan is probably, on average, much more accurate than LMP dates, and that's why they insist on using it. But there's always a chance they're wrong for any individual person, so who knows? All the later scans are less and less accurate, so they really shouldn't be changing your due date based on scans past 12 weeks. Growth scans can be way off, too.

Personally my LMP date was two weeks earlier than the scan date. But here I am, 3 days away from my scan date, and really glad that I had the later date to go by, because otherwise they'd be pressuring me to induce by now! I am quite happy to wait another two weeks if that's what it takes.

my scans with ds1 were definitely wrong.

i had a scan at '20 weeks' to date the pregnancy (don't ask!) but i felt the whole way through i was far further along than that and ds1 certainly looked and behaved like an overdue baby when i had him at '38 weeks'

same with dd.... i'm pretty sure the scan (also a dating scan ) was a week or so out with her too.

LMP date assumes that all women have a 28 day cycle and ovulate on day 14 (ovulation being the critical point as the egg will only survive about 24 hours after that point unless its fertilised whereas sperm can last best part of a week). Not all women have a 28 day cycle and not all women ovulate on day 14 - unless you've been doing fertility charting of some form and know when you ovulated then LMP can be out by a week or so.

Scan dates assume that all babies grow at the same rate for the first 12 weeks. They also have to make assumptions about tissue thickness and density, and hope that they get the baby at the appropriate angle to make the measurements accurately. IIRC some recent research indicated that the growth curves were only valid to 8 weeks and beyond this point growth rates start to vary on a baby by baby basis.

So neither method is perfect!

From what I recall of the growth curves there's quite a spread by the time you get into the last trimester. Ask to see how the measurements lie on the curves so that you can see how much difference being a week behind would make. Growth scans are notoriously innacurate, especially when they start estimating weight. That said, as longwayaway points out, there are advantages to having a "late" EDD!

Have you had a chance to sit down with a MW or Dr and talk through why they think you may have GD and what the implications for you care would be if you do? What would they recommend doing differently and what research are those recommendations based on? Its worth remembering that all procedures can only be performed with your consent - you don't have to go back for the next scan, you don't have to have any more blood tests, you don't have to agree to anything. I'm not advocating refusing all care by any stretch, but I do think a lot of women don't realise they have the option to ask questions and make their own decisions regarding their care before, during and after labour.