WOMENS-HEALTH Messages for November, 1997: Re: Accuracy of Second Trimester Ultrasounds

Re: Accuracy of Second Trimester Ultrasounds

From: Stormy (anonymous@obgyn.net)
Fri, 14 Nov 1997 12:32:40 -0600 (CST)

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In reply to: Mary: "Accuracy of Second Trimester Ultrasounds"

At Fri, 14 Nov 1997, Mary wrote: >
>I understand that early first trimester U/S is most accurate for dating
>a pregnancy and that late 3rd trimester U/S is less accurate for
>estimating fetal size.
>
>How accurate is second trimester U/S for estimating fetal size?
>
>My situation: U/S at 20 weeks - femur, humerus, hand, foot, abdominal
>circumference and BPD measurements all indicate 21.5-22.5 week size.
>(LMP and conception dates are known.) Fetal weight was estimated at
>16-18oz.
>
>>From what I have read, fetal size at 20 weeks is usually 10-12 ozs.
>
>Does the result of the U/S accurately reflect a large baby (or shall I
>say ANOTHER large baby - my son was over 11lbs)?
>
>Also, no indication of GD at this time. 3hr GTT in Sept was normal.
>
>Thanks for any opinions on this.
>
>Mary,

>From what I understand, ultrasounds done between 18-22 weeks are most
accurate for determining fetal size. When done at this time it tends to get around the big baby vs small baby variability that interferes later in pregnancy with ultrasound accuracy re: size. Realize that there is always a small percentage of error (up or down) in estimating. I don't believe that it is soon enough to say for sure that you are having another *big* baby though. You should have a baseline 3 hour GTT done asap (if you haven't already) to use for comparison as the pregnancy progresses. A regular routine GTT doesn't provide as much information. Some MD's also use a blood test called HgbA1C, or glycosylated hemoglobin, which tests blood glucose levels over the last 120 days, but is most accurate for about the last 60-90 days. With my own personal experience, I didn't show signs of GD unitl 22 weeks with my last baby and as late as 28 weeks with baby #3 so it should be monitored throughout your pregnancy.

Hope this helps!

--
  Stormy, RN, BSN

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