How accurate is an early ultrasound for dating? - Mothering Forums

How accurate is an early ultrasound for dating?

Mom2Adam's Avatar Mom2Adam (TS)
10:58 AM Liked: 10
#1 of 16
05-26-2008 | Posts: 484
Joined: Dec 2003
Hi -

I've been in the Jan. 09 club...but when I really started to think about when I last had a period...I realized it had been much, much longer than I had thought. My periods have never been regular (my whole life). They resumed when DS #2 was 16 months...but they would only come every 6 or 7 weeks.

Anyway...I had no reason to think i was pregnant until I started feeling sick...and sore breasts! LOL. last period was March 21st! Yikes...which makes me the end of Dec. if it was a 28 day cycle...but no way to know.

Okay...this is way too long winded...Sorry. Anybody ever had an ultrasound for dating? I did not do ultrasounds with my last baby and don't want to with this one...but know I need an accurate date...since I thought I was due late January.

Thanks -

Tenk's Avatar Tenk
11:04 AM Liked: 295
#2 of 16
05-26-2008 | Posts: 5,107
Joined: Oct 2006
I think early U/S are accurate as long as your past the 7/8 week mark. I only say that b/c I had one at 6.5 weeks and he was only measuring 5.5 weeks. We knew the exact date of conception b/c I was on fertility meds to get pregnant. AFter a few more weeks he was right on with measurements. Now at 31 weeks he's ahead a little. Good luck with your decision mama!
100%mom's Avatar 100%mom
12:18 PM Liked: 277
#3 of 16
05-26-2008 | Posts: 1,120
Joined: Jan 2007
My MW had me have an early ultrasound with my last baby even if I knew when my missed period was. That ultrasound was 5 days off. With my first at both my early ultrasounds the dates were within a day. This time around I had an early ultrasound at 8 weeks because I didn't even get my period back since my last baby.
caedmyn's Avatar caedmyn
12:42 PM Liked: 12
#4 of 16
05-26-2008 | Posts: 5,226
Joined: Jan 2006
I seem to remember reading that 8 week ultrasound are most accurate, to within a day or two (even if you know when your last period was, that doesn't guarantee an accurate due date since dates of ovulation can vary quite a bit). The longer you wait the less accurate an ultrasound is for dates. At 20 weeks they can be off by at least a week.
MamaSong's Avatar MamaSong
01:26 PM Liked: 11
#5 of 16
05-26-2008 | Posts: 510
Joined: May 2006
I had one at 7.5 weeks and it matched EXACTLY to my date of conception. I think I read that the earlier ultrasounds are more accurate for predicting due dates then the third thrimester ones based on size. But, the baby will come out either way - whether its December or January - if you aren't a planner it might be fun to wait and see
rstump's Avatar rstump
01:46 PM Liked: 10
#6 of 16
05-26-2008 | Posts: 709
Joined: Jun 2006
I have heard between 7-10 weeks is the most accurate. Only +/_ 2 days.
coyotemist's Avatar coyotemist
02:22 PM Liked: 19
#7 of 16
05-26-2008 | Posts: 1,761
Joined: Aug 2007
Because I'm high risk (history of 8 miscarriages and severe pelvic inflammatory disease) I've had 6 ultrasounds. My first one, turns out I was 4w3d, we couldn't see anything. My second one was 5w4d, I had a sac but no fetal pole, it dated me earlier than I could have been. Number 3 through 6 have all dated the baby consistantly (7w through 12w5d).

I'm done with ultrasounds now, I've just hit my second trimester!!! Whoo hoo!

It looks like by the last period date you gave, you are about 9 weeks. A dating ultrasound now should be fine, you should be far enough along to see a fetal pole, and maybe even a heartbeat (even if you ovulated a week or 2 late). Congrats!
Mom2Adam's Avatar Mom2Adam (TS)
03:22 PM Liked: 10
#8 of 16
05-26-2008 | Posts: 484
Joined: Dec 2003
Thanks for responses - it sounds like ultrasounds are somewhat accurate within a week or so...

I am not the type to worry about what day...I just let it happen...but there is a month's difference between Dec 26 and Jan 24. - Now that freaks me out a little! LOL.
AutumnMama's Avatar AutumnMama
03:46 PM Liked: 730
#9 of 16
05-26-2008 | Posts: 4,333
Joined: Jan 2004
Originally Posted by Mom2Adam View Post
Thanks for responses - it sounds like ultrasounds are somewhat accurate within a week or so...

I am not the type to worry about what day...I just let it happen...but there is a month's difference between Dec 26 and Jan 24. - Now that freaks me out a little! LOL.

Yeah, my Midwife thinks I'm a good 3 weeks further than I think I am...U/S is tempting with that kind of a range!
candyapplez's Avatar candyapplez
04:21 PM Liked: 0
#10 of 16
05-26-2008 | Posts: 890
Joined: Feb 2007
My mw said before 10 weeks is accurate for dating.
Nathan1097's Avatar Nathan1097
04:32 PM Liked: 13
#11 of 16
05-26-2008 | Posts: 3,360
Joined: Nov 2001
Hi. Not due in December, but I have not had a period "lining up" with my due date. My 2nd pregnancy, I KNOW I conceived during the 2nd week of January 1999, but my LMP was Nov. 5, 1998. Therefore, dating wheels said I was due in August, when I knew I was due in October. Ultrasound confirmed that I was right.
theboysmama's Avatar theboysmama
10:51 AM Liked: 25
#12 of 16
05-27-2008 | Posts: 3,364
Joined: Sep 2005
I think they are supposed to be fairly accurate but mine was a week off. It was a week behind my date and I gave birth the day before my date and I don't go early so I think my date was way more accurate.
juliav's Avatar juliav
12:11 PM Liked: 11
#13 of 16
05-27-2008 | Posts: 906
Joined: Nov 2007
I had an ultrasound at 9w2d (by my conception day calculation) because it didn't match up with my last period (my cycles are long) and it was bang on. The baby was measuring just a tad over an inch and looking just like a 9 week old should. I too have hear that they are very accurate in the 7-11 week range. Before an after that there is too much individual variation.
Good luck!
arahzel's Avatar arahzel
12:58 PM Liked: 0
#14 of 16
05-27-2008 | Posts: 473
Joined: Apr 2007
Early ultrasounds are so accurate because embryos grow at very much the same rate.

Once you get past past the embryonic stage and into the fetal stage, the beans start growing at different rates.
peace_momma's Avatar peace_momma
11:14 AM Liked: 0
#15 of 16
05-28-2008 | Posts: 50
Joined: Apr 2007
I had an U/S yesterday at my first appt. I'd been charting, not temps, just CM, sex, and periods to keep track. Since my M/C in Sept, I had two 42 day cycles, and two 38 day cycles, so we averaged this one at 40 and the doc actually adjusted the due date!!!! No having to lie about lmp. Anyway, she got the same due date I'd figured, 12/24. Then during physical exam, she said my uterus was measuring more like 16 weeks (what!!!) so we needed to do an u/s. After losing the last preg., I was relieved to see the bean and heartbeat, then the u/s tech says, no, you measure 8 weeks. WTH? Then I came home and looked at my chart and counted back 8 weeks and there was the day I'd picked for ovulation, and that still comes out to 12/24, with the whole, you're 10 weeks pregnant but the baby is 8 weeks thing. It was a very confusing hour and I'm just now coming out of the fog!
Swandira's Avatar Swandira
12:57 PM Liked: 0
#16 of 16
05-28-2008 | Posts: 902
Joined: Jun 2005
I was wondering about this too. I had a dating ultrasound in the ER when I found out I was pregnant, and I have no idea when I conceived. It said I was then 5w5d, and I've often wondered if I were actually further along (due to symptoms) or if this is just the way third pregnancies are sometimes.

mama to T, 5; L, 2; and EDD 12/20/08 -- if the u/s was right!
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