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Ultrasound accuracy

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Old 08-25-2006, 10:43 PM   #1
Join Date: Aug 2006
Posts: 4
kaijamaria HB User
Ultrasound accuracy

I'm wondering how accurate ultrasounds are at dating pregnancies. Is it possible to mistake a 7 week old for 11 weeks? Or is there usually a significant difference in size? Does the CRL size usually match up with accurate LMP records?

I'm pretty sure I had my last period on May 15th. I have not had any periods since then. But now I'm reading about people have implantation bleeding and I'm feeling kinda paranoid that maybe I've been pregnant longer than I realize.

I think I'm just paranoid, but I'd still like to get feedback.


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Old 08-26-2006, 05:49 AM   #2
Join Date: Jul 2006
Location: louisiana
Posts: 428
lovemybabyboys HB User
Re: Ultrasound accuracy

How far along are you? or do you think you are?

and what did the ultrasound tech or dr. say the baby measured?

i have one of those pregnancy calculators like they have at the dr. , and if the first day of your last peroid was may 15, then it says that you should be almost 15 weeks.

are you sure you have your dates right?

and are you counting your weeks the way the dr.s do?
they count form the first day of your last period, although you weren't actually pregnant until about 2 weeks later.

take care

Old 08-26-2006, 06:29 AM   #3
Join Date: May 2006
Location: US
Posts: 95
My5Cats HB User
Re: Ultrasound accuracy

I was SURE my LMP was April 1st of last year, so when I went in for an u/s I thought I was 10 weeks. But the doc told that the baby looked older than 10 weeks so she moved my due date up a week (which put the baby's age at 11 weeks) which the mathdidn't work for me at all...because that would have had conception taking place during my period (which, tmi, we don't have sex when I am on my period) I kinda knew that date was bogus, but I just went with it.

Well, DD was born 3 days after her due date, by c-section, and she had very dry, cracked skin...and more than one visiter asked me "was she post due?" and I kept thinking to myself, "no, if anything she is one week early!" Well, I got to reading about what some things are that affect post due babies...and now I am seriously starting to question if the doc was somewaht right (me being further along than I thought) and maybe the baby had been concieved a week to 2 weeks BEFORE what I thought was my last LMP.

I am starting to believe that what I thought was my last LMP was actually heavy implantation bleeding or spotting, and that DD was actually 2 weeks OVERDUE!!!

In my reading about ultrasound, they are the MOST accurate before 12 weeks because most babie grow around the same rate at this stage...but the farther along you get the more varied the development rates are. When I went in for a sizing ultrasound at 34 weeks and was warned that at this stage it can be 20% off either way.
Well, I hope that helps!


Old 08-26-2006, 10:28 AM   #4
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Join Date: Mar 2006
Location: Pittsburgh, PA
Posts: 155
jackiejetta_84 HB User
Re: Ultrasound accuracy

Same thing with me. I went in and my LMP was april 1st, which would have made me 17 weeks then (like a month ago). I knew that was the last time I had an AF for sure. But the baby only measured 12.7 weeks! That's like more than a month off! They even went on to scare the crap outta me by trying to get a h/b with a doppler, in which if I was 17 or 18 wks should have been picked up very strongly. But since I was only 12.7 weeks they couldn't get one. U/S's are a good way to "date" pg's. B/c you could have just ovulated later, like I did. I would believe the u/s over just when you had your last period. That's what I am doing. And since then I had another one now, 4 wks later and the baby measured 16 wks. So, the u/s was pretty right on! Good luck, and don't stress!


Old 08-26-2006, 11:55 AM   #5
Join Date: Aug 2006
Posts: 4
kaijamaria HB User
Re: Ultrasound accuracy

On July 7th I had an ultrasound and the doctor told me I was at 7 weeks. It measured 11mm CRL. No distinguishable limbs or head yet, just a little blob. So it does look pretty early in the pregnancy, but I really have no idea because I'm not a professional, so I'm hoping I can trust what the doctor says.

Yes, the doctor explained to me about how they count pregnancies conventionally from the first day of the last period. I thought that was a little strange, but it makes sense.


Last edited by kaijamaria; 08-26-2006 at 11:57 AM.

Old 08-26-2006, 05:25 PM   #6
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Join Date: Aug 2005
Location: Panama City Beach, Florida
Posts: 1,698
xinerevelle HB User
Re: Ultrasound accuracy

Or it could be due to delayed implantation. Sometimes it takes more than the typical amount of days for the ovum to implant in the uterine wall, which can cause the dates to be a little off. And of course, the fetus grows at differing rates, so things can always get screwy along the way!

Old 08-26-2006, 05:40 PM   #7
Join Date: Jul 2006
Location: louisiana
Posts: 428
lovemybabyboys HB User
Re: Ultrasound accuracy

by no means am i a professional either, but like i said i have one of those pregnancy calculators that they have at the dr. office and i am looking at that, and that sounds right according to your last period. if your last period was may 15, then on july 7 you would have been 7 wks 4 days, and now you should be 14 wks. 5 days. so it sound like the baby is measuring just right.
take care, try not to stress, and remember that babies do grow at different rates.

Old 08-26-2006, 05:45 PM   #8
Join Date: Jul 2006
Location: louisiana
Posts: 428
lovemybabyboys HB User
Re: Ultrasound accuracy

i just re-read the post, maybe i am misunderstanding,
what does CRL stand for?

Last edited by lovemybabyboys; 08-26-2006 at 05:45 PM.

Old 08-26-2006, 06:18 PM   #9
Join Date: Aug 2006
Posts: 4
kaijamaria HB User
Re: Ultrasound accuracy

I think you're reading it right. CRL stands for Crown Rump Length. Basically, it's the length from the head to the butt, since at that point the limbs aren't even visible yet.

I've read that usually before the 13th week, babies pretty much all grow at the same rate and are all close to the same size, which is why ultrasounds are so accurate early in the pregnancy.

Thanks for all the feedback.

Old 09-04-2006, 05:38 PM   #10
Join Date: Sep 2006
Location: California
Posts: 91
writekaren HB User
Re: Ultrasound accuracy

When I had my ultrasound, my due date was moved WAY up. I apparently spotted for two periods because I had thought my due date was Feb. 11th, but found out it is December 20th. I knew it had to be sooner since I was feeling strong movement, but didn't expect that much.

Old 09-11-2006, 09:51 AM   #11
Join Date: Jul 2005
Posts: 57
renau2e HB User
Re: Ultrasound accuracy

I just stopped taking birth control when we concieved our baby and didn't have a normal period before hand ... so I had an ultraound done very early to date the pregnancy.
The tech told me that I was only 6 weeks 5 days along. Being that I had no clue when I concieved, I stuck with it.
Then, I had my second ultrasound around the end of my 12th week and the tech told me that the first results were wrong, giving numerous reasons why, saying that I was actually 14 weeks 2 days! 1.5 weeks ahead of what we thought!
My 3rd and 4th ultrasounds, along with my measurements, have all agreed with that second U/S ... so in fact, when thinking I was less than 7 weeks along at the beginning, I was actually over 8 weeks!

Mistakes happen that early on because the baby isn't very clear on the screen and looks like a huge blob. If it's lying in a certain position, it's easy to measure incorrectly (so the tech told me)

Hope this helps,

~ Sarah ~

EDD- Jan. 6th, 2007 (1st baby!)

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