How accurate are dating scans?

I had my dating scan today, and they said I was exactly 9 weeks along. Due on the 11th March. I'm pretty sure that my last LMP was correct with what I thought it was, and that my dates are correct.

How accurate are dating scans? Can they be 4 days out? I'm wondering since they take it from the baby's measurements, and I have super long babies anyway (thanks to DH) then maybe thats thrown it out a little, but all my other dating scans for my other kids have all been spot on - to the day.

I really don't think I'm wrong though.... could it make a difference that this scan was done by a totally different place, with a different person? All my other dating scans were done at the same place by the same person on the Sunshine Coast, so I guess I'm wondering if its possible the person who did mine today can get it wrong?

I know 4 days isn't much.. but when you usually go over by 2 weeks anyway... that could be the difference between a repeat c-section and another natural birth....:pray:

my midwife and many others have told me ur very first early scan is what they generally go by wit those dates :)

Goodluck either way!

It's a week either way I think. I'd go with your date if you're happier with that - my scan date was 6 days later than cycle date so we're going with the later one. It hardly makes any difference anyway till you're staring down an induction.

Hey Melski! :)
I queried this with DS too because, same as you the matter of a few days can make a huge difference when you are heading towards at VBAC/VBA2C and what I was told something along the lines of, a dating scan any time before 12 weeks can be around 4-6 days out, and the earlier it is, the more accurate it is which adds up to what you are saying. Mine was at 7w4d. As you know he was 14 days over and weighed 10lb so I still dunno how accurate it actually was. Are the 4 days the "wrong" way around?? Like if you go 14 days OD would it be 18 according to your dates?? Tricky one babe.

Hi, my original dating scan said the 4th May was the due date for my DS. Subsequent scan changed it to 2nd May....bub arrived on 4th May - but my waters broke on the 2nd! Pretty accurate for me - but my dates also were the 4th according to my estimates from ovulation :-)

Thanks guys

Tan - its the "wrong" side of it, so if I go 14 days over by MY dates, I would be 18 days over for the dating scan date. :(

A quick run down of my last 4 pgcy's. (incl this one)

DD3 - I went by my dates, as my bubs tend to start of smaller. She was out by about 6days. She was born on her due date, my dates.

DD4 - I was told that I was due on the 12th June, I kept telling them that I know when I fell pg and that she is due the 29th May. They wouldnt listen to me even with the proof I had with finding out that I was pg BT results. They had me out by 2weeks. I kept telling them that my bubs start out small. I had a feeling I would go early, they had booked a c/s the day before my due date, 2wks earlier to their dates. Bub arrived 2wks earlier from my due date, the 15th May. To my amazement after all the stress they put me through they ended up putting her down as being born at 38wks not their 36wks.

DD5 - Again she started off small. They had her out by about 5days at first, again I knew when I was due, so it didnt really bother me as much this time as it wasnt as bad as the previous pgcy being told 2wks out.

This time, they again have me out by 5-6days. Although my NT scan and the morph scan both show my dates, yet the stupid Dr at the hospital wants to leave it at the first scan, even though my bubs start out smaller. So anyway, Im sort of thinking it may work a little in my favour this time, as I dont want a c/s too early, would liek to get as close to my dates as I can. lol.

Now my first two, were spot on with my dates, and one was born a few hours before his due date and the second was born a few hours after his due date. They normally give it 7 days either side.

Mine has changed a few times, I had myself due on the 27th based on LMP, I charted my cycles for 6 months with 4 or 5 at 25 days and the one before conception being 26 days.

At first I was put down as the 23rd of Oct, based on the early scan, then everything went to crap, and after many weeks my final EDD is now the 30th of October :doh: which I guess is giving us a bit of extra room with the growth scans atm ;)

Maybe talk to them about the fact that they can be out, no scan will be 100% accurate but you know your dates and cycle length and the fact that you don't want to be thinking it's time to make a move about it all if you still have 4 days to go? Sounds like they are incredibly supportive hun so talking to them about your concerns can only be a good thing. Maybe they will agree to monitoring up to a point where you guys are comfortable? Good luck hun.

Have you booked with a midwife yet? She's very likely to go with the 'better' date given your history, I would think. I told mine the same thing - 6 days difference and wanted the later date since I was induced at 42 weeks last time. She agreed that would be sensible.

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Have you booked with a midwife yet? She's very likely to go with the 'better' date given your history, I would think. I told mine the same thing - 6 days difference and wanted the later date since I was induced at 42 weeks last time. She agreed that would be sensible.

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The midwife down here didn't want me to get in touch until I was 16 weeks. However, there is a chance we *might* be moving back to Queensland in January, so I have also been in contact with my old OB on the Sunny Coast - and she is really supportive, however, don't know how willing she would be to go with my dates - ITMS?

Well if you end up moving, maybe just only tell her one date...?

But she'd see the date on the dating scan report - unless I don't give it to her... :think:

My scan-dates were only a max of two days of the calculated dates, and the further the pregnancy progressed, the less that happened. At 20 weeks, and 35 weeks, Jack was spot on for dates. Didn't stop him being late - but that could have had more to do with me (??dodgy pelvis?) than him.

Tell her it got lost in transit - or you didn't have one :D Could you have ovulated late / early?? I did with DS2. Really, really early positive POAS but I know when we conceived (and where :lol:) so it was a later ovulation despite fertile signs earlier (twice in the cycle). My scan dates were 6 days later than my original dates.

OH MY GOSH MEL :jawdrop: are you pregnant again!!! how could i miss this??? sorry! and CONGRATS!!!!!

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Could you have ovulated late / early?? I did with DS2.

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Hmmm. Ok, well my CD 14 would have been on the Tuesday, and we DTD on a sunday I think (from memory, it feels like it was a sunday) so that would have been CD12 and I definitely had EWCM. Thats the only time we DTD for that cycle (lucky we got to do it once really)... so does that mean it would be taken into account, meaning its out by only 2 days?

My head is spinning.. LOL

HJ - thanks ;)

4 days isn't too much? - im measuring 3 days ahead for my gestation - i had my scan on wednesday and at that stage going my lmp i was 11wks 3 days, but by the measurements on my scan i was 12wks and 2 days, so not really sure? - im 100% sure when i ovulated so i must just grow bigger bubbas as even at my 8wk 5day scan at my ob's i was measuring 9 weeks 4 days :)

With DD3, my dating scan was out by 12 days according to my LMP dates. For me though, it wasn't really a problem as I never reach 40 weeks anyway.
If your dates are only out by 4 days, I wouldn't worry too much. If you do transfer from Tassie to Qld though, wouldn't your Qld OB request your file from your Tassie OB? Then he/she will know your dating scan dates??

Nah 4 days isn't much, and if I was someone that had my babies on their due dates, it wouldn't bother me.. but the difference of being able to go 14 days over or 18 days over... could mean a lot... ITMS.

i had a n early dating scan with DD2 at about 6 weeks (from memory) i ended up being induced 10 days after my 'due date'. so it wasnt accurate for me.
btw, congrats mel, another bubba on the way!

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If you do transfer from Tassie to Qld though, wouldn't your Qld OB request your file from your Tassie OB? Then he/she will know your dating scan dates??

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I don't have an OB down here. No OB would touch me with a ten foot pole for a VB, so I'm just going to my GP for my check ups.

I never changed my EDD as i knew when things happened due to IVF, but at one point they did measure me three of 5 days ahead, but by the time i had my next scan thongs were measuring on first date again.

I dont think i would have changed the date anyway, i mean its not like the baby WILL be born on that day! lol

I would think that your files would all be sent up if you guys move so it will all be in there anyway? Would your old ob be prepared to negotiate based on your concerns and dates hun? Knowing that 4 days for a VBA2C is actually alot and can mean the difference between a VB and C/S? Is she the one who would let you go past 14 days anyway?? Hopefully bub won't be a cheeky possum pants and make you need to worry anyway but best to be prepared. :hug:

i am asking my self this question myself...i was religiously charting my cycle cause of the difference between dates i was given last time, literally a week between them, not a problem till they think your 10 days over and you think your only 5 days over!
So i wanted to get it right this time! The first day of my last period was 25th May and i oed on the 8th/9th/10th June. so by my calculations due on the 22nd feb. but my doc says im due on the 28th and the EDD calculator says the 2nd march.....confused much!!! im sure the scan will tell me a different date!
id go with the one that puts your edd at the last possible date.... :)

Oh - I forgot, my LMP date was wrong because I had IVF and I ovulated late that month (day 20... what's up with that). But I used the embryo tranfer date, back dated to ovulation date. I even had a disagreement with my OB about when I was due... scan says: I was right! :D

I totally understand where you're coming from and think you should just insist. For them it's only 4 days, and really shouldn't be here or there unles there's some problem with the pregnancy. For you it could be a very important 4 days and the difference between a VB and a repeat C/S.
With both my pregnancies I"ve gone with the eearly scan date even though we did IVF and know exactly when conception ocurred because I really wanted those extra days - I think it was 3-4 days the first time and 6 this time. I figure this time round I have till nearly 44 weeks before I'll be marched off to the hospital (I hope it won't come to it of course).
Anyway, some docs have a bias toward LMP/O/cycle dates and soem prefer scan dates, but in the end they're your dates and it's your pregnancy - you know your dates and you're confident in them and that's that.