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How accurate are dating scans?

(28 Posts)
PixieCake Tue 11-May-10 17:34:46

I've split this question out from an earlier thread...

I've just been for my 12 week scan and they have told me I am 15 weeks pregnant. This simply does not work out - I am sure of my dates and even did an ovulation test to check when I could conceive that month.

They must base the measurements on averages, but 3 weeks out seems like a lot. Has this happened to anyone else? If so, did your dating scan turn out to be right according to later scans? Or might they tell me later on that my dates were correct after all.

Can some babies just grow faster than others? 3 weeks faster than average?


thisisyesterday Tue 11-May-10 17:36:16

i think they're fairly accurate, they all tend to grow at similar rates in the early weeks, as I understand it

belledechocolatefluffybunny Tue 11-May-10 17:37:28

Are you measuring your dates from the first day of your last period? It's really common not to do this. A scan date is normally spot on although if there's a few days difference between the scan date and your date they will go with your dates.

londonmackem Tue 11-May-10 17:39:01

I think mine was spot on, 12 weeks 2 days was actually 11w6d which I agreed with but they are not infallible. Doesn't really make any difference as my daughter was 3 weeks early.

RooBear Tue 11-May-10 17:46:23

I got told it was usually correct to within a week either way

PixieCake Tue 11-May-10 17:47:32

Yes I am measuring it correctly - I was a bit obsessed and recorded everything in my diary. They said that my last 'period' might have just been spotting but I'm sure it wasn't.

Everything ties in with my dates (first day of last period, date of ovulation test, date of getting jiggy (!) to coincide with positive ovulation test, pregnancy test, and even a few spots of what I think were implantation bleeding).

I am simply baffled that the scan shows the baby to be 3 weeks bigger than it should be.

DH is starting to shoot me suspicious glances!!!!

belledechocolatefluffybunny Tue 11-May-10 17:53:37

Ahh, I see your problem. You could just have a long baby?? They can make mistakes at a 12 week scan as the baby wriggles about alot.

EdgarAllenPoll Tue 11-May-10 17:57:19

at best +- ten days where done at ten weeks.

HappyGirl1 Tue 11-May-10 18:26:45

I'm in a similar position.. I was scanned at what I thought was 8 weeks 4 days.. I was sure of this as I had MC'd the previous month unfortunately, done ovulation tests etc and then when they scanned me they said I was 7 weeks 2 days which just makes no sense given when I ovulated and I'm constantly worried about it now. I just hope baby is developing okay. MW said it was fine and you never know how the body works ovulation wise though.

Then again I've read that at 7/8 weeks the baby is so small that if they are out by a few millimetres then they can end up being out by a few days.

got another scan at 12 weeks so am really hoping they will tell me everything is okay.

fabhead Tue 11-May-10 18:31:14

i can see how you can be less pg than you think as the sperm can hang around for 5/6 days apparently before doing the deed. Harder to see how you could be more pg than possible from jiggy days - are you sure ther weren't any drunken ones you've forgotten about smile

PrivetDancer Tue 11-May-10 18:35:59

Early scans can be out by 5 days or so but I thought by 12 weeks they were regarded as fairly accurate. 3 weeks is a big difference! I don't think they would be that far out.

Perhaps you ovulated twice that month?

ande Tue 11-May-10 18:46:36

I was sent to EPU as was being very sick. By my dates I was 7 weeks, by theirs I was 6 weeks. The sonographers explained that the earlier the scan the more accurate as the embryo has very standard development. I had one embryo with fetal pole and heartbeat and one blighted ovum. The sonographer explained that indicated that this was a twin pregnancy with only one twin viable and could explain the date confusion as I may have ovulated twice in one cycle. (I already have twins)


ps Congratulations!

EdgarAllenPoll Tue 11-May-10 18:51:43

i meant to add: dating scans are more accurate than LMP, as you can ovulate any time in cycle...(although certain days are more likely) and sperm can hang about 7 days...

caen Tue 11-May-10 19:20:10

Being pregnant can give a positive result on an ovulation test because the hormones are similar chemically (as I understand it). So is there any chance you could have been pregnant when you tested for ovulation?

ktwiltshire Tue 11-May-10 20:17:28

^ i was about to come in and post that!

PixieCake Wed 12-May-10 09:50:37

Thanks everyone. So it sounds like the scans are pretty accurate then.

I did tell the Dr about the positive ovulation test and he didn't tell me that it could have been the pregnancy hormones, but that sounds like the most likely explanation.

I'm still not entirely sure what can have happened with the dates, and I definitely had a period (thank god I didn't know I was pregnant or I would have thought I was having a MC), but I suppose I have got an extra 3 weeks of pregnancy 'for free' now!

jade1245 Sun 26-Feb-17 12:46:00

hi I currently went for an early scan on the 2nd Feb and got told I was 6 weeks 4 days.. I then went for another scan on the 22nd Feb and got told I was 9 weeks 1 day. I slept with someone on the 21st December and have spoken to my doctor about this and she says sperm can only live up to 3 till 5 days so the time frame misses it as I would of fell pregnant beginning of Jan. the thing is I don't remember when my period in December was so I can't calculate my overlation day I'm so stressed I can't sleep I'm crying all the time as I need to work out if it's my parents or the person I slept with. how accurate are scans if I am 9 weeks 1 day on the 22nd then it can't be the other guys but I've read it can be a week either way I'm just so confused someone please help?

BHillary Wed 15-Mar-17 12:35:32

You can't be 100% sure ok ovulation dates as they vary from month to month.
If you know exactly how many weeks you are then count back.
Do a conception calculator and out your due date in and then you will know smile

anxiousandpregnant Wed 15-Mar-17 13:52:27

I went for what I thought was my 12 week scan..... I thought at the time I was about 11 + 4 (worked out from my last period) and I was actually only 9 + 3!

No idea how it all works to be honest.

Littlecaf Wed 15-Mar-17 14:08:29

While they are probably generally accurate, is suspect there is room for error in measuring, the sonographer is only human!

For example I know when 'it' happened, and therefore I'm 11+3 not 13+5 as yesterday's scan told me!

BHillary Wed 15-Mar-17 14:49:56

Yea but you add two weeks So it's from lmp
So if you did "it" 11+3 weeks ago a scan to say 13+5 is about right and only 2 days out

MrsCharlesBrandon Wed 15-Mar-17 15:21:27

BHillary it would be the other way round. They assume ovulation happens 14 days after your period, so going by lmp dates you would be 13+5. From ovulation would be 11+3. 2 weeks is a long time in fetal development terms so it's more likely that ovulation happened closer to the period than actually thought. I would expect a 13 week fetus to be significantly bigger than an 11 week one.

Going by my scan with DD1, I concieved on the first day of my period. DD2 was 3 weeks after, and DS was 3 days. This is all while using ovulation tests. (PCOS here so anything is possible)
Scan dates are generally thought to be more accurate than going by period dates.

MrsCharlesBrandon Wed 15-Mar-17 15:23:35

Sorry, ignore my waffle. LMP would be 11+3, and ovulation 13+5.

Having a daft day today!

BHillary Wed 15-Mar-17 15:43:52

Yeah in inderstand that what I meant was if the lady thought she was 11 weeks from conception then went for a scan and was 2 weeks ahead than she thought it would be because he scan would have gone from lmp as he first 2 weeks of pregnancy you aren't actually pregnant smile

BHillary Wed 15-Mar-17 15:45:52

For example of a clear blue didgital says 2-3 from conception weeks you're actually 4-5 weeks pregnant. Does that make sense?

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