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Scan dates can be out by 15% apparently, but 15% of what? - pedants?

(24 Posts)
StarlightMcKenzie Fri 05-Sep-08 18:28:08

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Piffle Fri 05-Sep-08 18:31:31

Dd 2 wks early
Ds1 2 wks late
Ds2 1 day before edd
Scientific huh?

LaDiDaDi Fri 05-Sep-08 18:34:06

MY guess would be that the meaurements upon which they base the dates could be out by 15%. Not sure how this would translate to days though.

StarlightMcKenzie Fri 05-Sep-08 19:33:47

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DistractionTechniques Fri 05-Sep-08 19:44:25

<grasping at straws> If 40 weeks were 100%...

DistractionTechniques Fri 05-Sep-08 19:46:27

Hmmmm, maybe. It works out to six days.

maretta Fri 05-Sep-08 19:47:31

How does anyone 'know' anyway. Have they got some more accurate measure of dates to compare the scan dates too.

And 'scan dates can be out by 15%' in isolation makes no sense at all.

maretta Fri 05-Sep-08 19:48:01

Sorry should be 'to'. This is a pedants thread.

DistractionTechniques Fri 05-Sep-08 19:53:45

maretta - They could evaluate the accuracy of the due date predictions by comparing the predicted DOB with the actual DOB.

bethdivine Fri 05-Sep-08 19:59:04

I read that scans are accurate to within 5 days. In the first 12 wks the baby's growth is extremely predicatable, but they can only be accurate to within 5 days.

StarlightMcKenzie Fri 05-Sep-08 20:48:46

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expatinscotland Fri 05-Sep-08 20:51:49

erm, 15% in either direction?

sympathies.

this baby is HUGE. our HV was a midwife in the consultant-led hospital where i'll have to give birth if anything goes wrong, so i let her have a feel, too.

she said she'd be surprised if i went to term.

so do all teh community midwives at the MLU.

we'll see what consultant thinks when i see her next week.

he's not go extra fluid but he's above the line on the growth chart - not enough to be of concern.

DistractionTechniques Fri 05-Sep-08 20:53:15

Good point Starlight. I forget that's the rule in the UK. It's 43 weeks here, which means pretty much everyone gets to wait unless there's a specific risk.

WendyWeber Fri 05-Sep-08 20:55:02

My scan dates were always ahead by 2 weeks - I knew my own dates but they ignored me hmm (I never went into labour so no way of disputing it)

Agree that "out by 15%" is a meaningless statistic (15% of 40 weeks is 6 weeks & I bet none of them are ever that far out grin)

InTheDollshouse Sat 06-Sep-08 18:52:44

I think that studies on ultrasound accuracy look at pregnancies where the timing of implantation is known, e.g. pregnancies resulting from IVF, and see how the EDD from scan measurements compares with the known EDD.

StarlightMcKenzie Sun 07-Sep-08 09:09:22

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SueW Sun 07-Sep-08 09:15:56

Since term is considered 37-42 weeks, I sincerely hope they aren't thinking of adding another six weeks either side of that!!

lollipopmother Sun 07-Sep-08 12:28:07

But if you add 6 weeks to 37 then it's not far off, maybe that's where the 37-42 comes from? I'm thinking not though!

WendyWeber Sun 07-Sep-08 14:37:10

A total of 6 weeks out, 3 weeks either side, would be 7.5% - maybe the person who came out with 15% isn't a mathematician & just added the 2 7.5%s together?

(Like Roy Meadow & his 1 in 73 million hmm)

InTheDollshouse Sun 07-Sep-08 19:39:07

Yeah, dunno Starlight - where does the 15% figure come from? Where've you seen it?

lindenlass Sun 07-Sep-08 20:33:58

The scan dates predict at what date you'll be 40 weeks pregnant, and when they call it an 'expected date of delivery' they're actually being really inaccurate because human gestations are between 37-42 weeks long and you're only overdue after 42 weeks. Plus, the scans are + or - five days out, but, like I say, they only predict the date you'll be 40 weeks pregnant - which is why it's so annoying when they choose the scan date over a woman's dates even if she knows the date she ovulated!

Anyway, does that help a little?

Notanexcitingname Sun 07-Sep-08 21:22:39

I think where it comes from is that the scan dates are calculated from the crown-rump length (CRL) of the foetus. And it's assumed that the foetus will be on the 50% centile (bit of an assumption!). The actual variation of size at that gestation is known (not sure how!), and the difference between these extremes and the 50% centile is approximately 15% of the number of days gestation.

Unless we're talking growth/size scans in later pregnancy, which seem to be incredibly unreliable!

Elkat Mon 08-Sep-08 23:02:30

I was told when I had DD2, that scan dates are very reliable and if I queried my scan, then it was probably because I did not know my cycle very well etc. I thought that was a bit hmm because whilst she was right , I don't know my cycle, but I did know exactly when I got pregant - and my scan date was wrong by a week. So not sure if that helps at all??

Elkat Mon 08-Sep-08 23:03:28

sorry, at least a week.

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