ultra sound due date accuracy

[beka277]

Hi, just wondering how accurate an ultra sound is at about 17-20 weeks pregnant.

According to my last period in last week of September 2007 my due date is the 25th of June 2008. I had an ultra sound when i thought i was 20 weeks pregnant but it changed my dates, telling me i was only 17 weeks pregnant and due on the 14th of July 2008!!

It has put my dates out quite a bit and as my due date is approaching fast Im just wanting to know when Im actually due!!

Did the same thing happen to anyone else? Whats everyone think?

Wow, that's a big difference. My dd chamged at this scan, but only by 3 days.

I had exactly the same but the other way around. My last period was also Sept 07 with gave me a due date of 25th June. I then had a 12 week dating scan and they moved my due date to 15th June. My midwife said that if the two dates are over a week different they work 40 weeks backwards from the due date rather than forwards from your cycle date to calculate how many weeks you are if that makes sense. Hope that helps although quite honestly I am a little bit confused as to when my due date is now.

Hi Beka, this is spooky, this happened to me too, and with almost exactly the same dates! LO is due 15th July now instead of 29th June as I had thought by my periods. Must have been something in the air that week!

Personally, I think the ultrasound people were right for me, as I have measured right for their date ever since. How does your baby measure? I know that's not necessarily a reliable predictor, but 3 weeks out is a big gap.

Still bloody weird though, I can't imagine how I managed to conceive then ...

Dating later on in pregnancy is not very accurate. Below 12 weeks the fetus grows at the same rate, after 12 weeks there can be a difference.

An experienced midwife is your best bet. The midwife i trained with was amazing, she could put her hands on a bump and know how many weeks they were. She was always getting me to do it as a test!

I would ask your midwife what she thinks A later date is better as it means you will be all surprised if the baby comes early.

My dates were questioned right from the 12 week scan. Baby was far bigger than they thought he should be. They kept giving me wildly different dates. Turned out that MY date was the correct one, and he popped out at 9lb 13oz. He was always going to be bigger than they thought he should. In fact he still is! He's going to be a big man.

Big difference in dates. My dates were only changed by 3 days too.

My EDD was worked out by me and then the midwife as 11th June. Then the scan revealed a large baby, etc and they changed it to the 1st June. However, as I am still pregnant and showing no signs of anything else happening (dammit!) I am guessing the original EDD was more accurate...?

With my first baby, i knew i was due on 5th december (due to only having had sex once in the weeks before conception), scan gave me an EDD of 21st Nov, the took me in to induce me on 2nd dec and he was born on 3rd Dec. He was 8lb 12oz - apparently i am to assume i have large babies!

2nd baby, i knew was due 6th Feb, scan gave me EDD of 21st Jan, they gave me a C-section on 8th Jan - i was very worried because i KNEW my dates were correct and my baby would be born at 36 weeks. She was 7lb 14oz.

3rd time round, i had a scan at 5weeks which showed an egg sac, had another at 6weeks and they dated me at 6weeks and 3days . had a "dating scan" at 12 weeks and they dated me at 13+4 meaning baby was conceived 2 days after my OH had a hernia operation and was still in hospital!!!

Thus proving not all scans are accurate! Maybe you're having a big baby!

Thanks for all your comments!

My fundal height measurements are as follows.

13 wks (ultra sound) 15 wks (my dates)= 16 cm
17 wks (ultra sound) 19 wks (my dates)= 18 cm
21 wks (ultra sound) 23 wks (my dates)= 26 cm
27 wks (ultra sound) 29 wks (my dates)= 33 cm
32 wks (ultra sound) 34 wks (my dates)= 32 cm

So i have been measuring bigger till my last checkup at 32 weeks! Not sure how accurate measurements are either though...?

I can't believe you girls had the same thing happen to you, definetly must be something abt September...
Im not sure wht you mean Lolski, abt how to calculate the due date backwards...? Ive neva heard of tht..
ANy more input would be great!

Hiya

SAme thing with me, told we were due in June then told at 20 week scan that we were due in August.

Measured 35 weeks today by the doctor and was informed that by 2nd JUly (my next midwife appointment) I would prob already have a baby! lol

Sorry Beka. It is a bit confusing isn't it. I will try and explain a bit better. My Cycle due date was 25th June but my dating scan due date was corrected to the 15th June. My MW said that because the two dates are more than a week apart you have to calculate it as follows:
Instead of taking the first day of my last period which was the 19th Sept and adding 40 weeks to give you a due date of 25th June you have to start at the new dating scan due date of 15th June and deduct the weeks starting from 40 backwards to get how many weeks you are. So basically I was told to keep taking weeks off from the 15th June until I get to the date now and that is how many weeks I am. Does that make sense ? Quite honestly I am confused myself. I am 39 + 3 at the moment and nothing is happening. The MW has booked me in for a sweep at 41 weeks and I am a bit worried that it will be to early to do it in case the original date was correct

Lolski, how absurd tht our period was on the same day and I think I'm 35 + 4 and you think your 39 + 3 !!! SERIOUSLY, tht is so strange. That is one months difference in due dates, which surely is impossible??

Do you know how many days your cycle is? Maybe you have a bigger than average baby?

Lets look at the worse case scenario - say u r really due on 25th June, this would mean the 15th June puts you 10 days early, which would mean if u hav the sweep at exactly 41 weeks then u would only be 39 weeks 4 days. Which I guess isnt so bad, the baby would be mature enough to be born healthy. . .
Worse case scenario for me is that I could go 10 days past my scan due date which would be 24th July which would put me 4 weeks past my original due date!! Ahh lets hope this baby doesnt over cook! bit scary.
Goodluck with it, you r very close (which doesnt seem fair to me, hmmmm) LOL

Hi Beka. I know it's really confusing isn't it. One thing that my midwife did say was at the 12 week dating scan (which is when I had mine done) it is a very accurate way to tell the date because all foetus' grow at the same rate. After about 17 weeks apparently genetics kick in and so babies will start measuring different sizes !!
Good Luck anyway. One thing that we can both be sure of is they they will both come eventually

I'm afraid I don't have an answer for you but wanted to share my experience which is from a slightly different perspective.

I conceived using IUI so my dates are absolutely certain. Because I had been having fertility treatment I had more scans than usual and my dates by the scans vary every time...

at 8 weeks scan was 8+3
at 12 weeks scan was 12+6
at 18+3 weeks scan was 19+6

So you see, my baby seems to be getting bigger every time we go. I think that provided you were having a regular cycle when you conceived I would stick with your original dates as in my experience the scans can vary considerably

thanks for tht beaufies, very interesting info...

Only thing is it is weird to me tht if my cycle dates r right, this means im carrying a small baby by the ultra sound. My first baby was 9 lb !!