Confused after dating ultrasound... « Back to Group Discussions

[Cat77]

Here's my little bambino!

So, on Monday I went for my first proper ultrasound, when by my calculation I was 11 weeks and a few days - this reached by counting from the date of my last period, (I wasn't charting or anything to know ovulation dates or anything like that).  

At the end of the u/s the tech says I'm 12 weeks and some days, essentially a week more than I thought.  I didn't ask at the time but now I'm a little confused - am I to take it to mean that I'm farther along than I thought, or that baby is 'measuring ahead', or is it too soon for that? Or am I one of the rare people that may've conceived before or during my last period?

Sorry if my question seems silly... I don't see my OB til the middle of next week.  Anybody have a similar experience?

[odinsmama]

I'm guessing that would be the same boat I'd be in, if I had an u/s.

Based on my last period I'd only be about 12 weeks, but based on when I know I ovulated (the day after my period stopped, so on the 7th or 8th day of my cycle), I'm counting it as 13 weeks.  And I know that most practitioners typically estimate your due date based on your period, so they'd be telling my I was 12 weeks and then changing to 13 after a dating ultrasound.  Does that make sense?

You could have just ovulated during or right after your period.  Or it could be that their measurements aren't perfect.  :)

Either way, with any of the calculations... last period, ovulation day, dating ultrasound... it's all just an estimated date and baby will come when it's time!  :)

yay for a picture of your babe!

[jenifera2]

Dating ultrasounds are most accurate earlier in the pregnancy than you are now. Your results would mean that you ovulated earlier than typical in your cycle, as odinsmama said, or more likely that you fall within the accuracy band and possibly had a slightly off measurement. A millimeter makes a difference! I'm fairly certain of my ovulation date and had an ultrasound at 11w4d. The first measurement measured 12w4d. The second, slightly different view, measured 12w1d. Definitely within the fuzz band of accuracy this late in the pregnancy. It doesn't matter a whole lot, but if your doctor starts giving you a hard time about inducing based on the ultrasound dates, keep in mind your original dates and don't be bullied beyond your comfort zone!

[Cat77]

Thanks for replying odinsmama.  I don't know what I really expected anyone to say, but I had to get it off my chest.  I know none of this really will affect the day my  baby decides to make its appearance, its more me being anal about knowing exactly how far along I'm supposed  to be - now I feel a little unsettled.  Anyway, thanks for sharing.

[becalive]

just remember each baby is different! even at this young age..... i just got my 10 week ultrasound and compared it to my friends last baby at 10 week ultrasound and her baby measured at 14 weeks, even though she knew exactly when she got pregnant...... sure enough her baby was came on its original due date and not a month early. She did have a 11lb baby though but all her babies have been over 10 lb. 

[Cat77]

Originally Posted by jenifera2 

Dating ultrasounds are most accurate earlier in the pregnancy than you are now. Your results would mean that you ovulated earlier than typical in your cycle, as odinsmama said, or more likely that you fall within the accuracy band and possibly had a slightly off measurement. A millimeter makes a difference! I'm fairly certain of my ovulation date and had an ultrasound at 11w4d. The first measurement measured 12w4d. The second, slightly different view, measured 12w1d. Definitely within the fuzz band of accuracy this late in the pregnancy. It doesn't matter a whole lot, but if your doctor starts giving you a hard time about inducing based on the ultrasound dates, keep in mind your original dates and don't be bullied beyond your comfort zone!

Interesting, I didn't know that.

Thanks for your input too becalive :)

[Teles]

Another thing to keep in mind is that ultrasounds before 20 weeks have a range of error of around 1.2 weeks on either side of your due date.
 

[Cat77]

Thanks Teles, didn't know that either.  Good to ask seemingly silly questions sometimes I guess. 

[jenifera2]

Here's one link that discusses dating briefly:

http://library.med.utah.edu/kw/human_reprod/lectures/clin_radiology/

It indicates that between 6 and 10 weeks, ultrasound dating is accurate to +/- 4-5 days, which is considered "very accurate" by most other sources I've read. Before 6 weeks other indications (not crown-to-rump length) can more accurately indicate embryonic age. Once the embryo becomes a fetus, growth rates start to vary considerably (which is why you can have a baby at 40 weeks be 6 pounds or 10 pounds!), so measurements are not as accurate when compared to averages. If there are concerns they can still chart your own baby's growth over time to see that it's changing appropriately, but just can't compare to other babies to determine age, at least not very accurately.

[Cat77]

Thanks jenifera2!