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OK experts - ?? about ultrasound accuracy

476 views 2 replies 3 participants last post by  mothercat 
#1 ·
I was told by the US technician that before 12 weeks gestation all young fetuses develop at the same rate and EDD determined during this time period are very accurate. (I was found to be 10 weeks, 1 day) Is this really hard science? I've since had some "off" measurements, from measuring small in the beginning to measuring large now (slightly above belly button, 21 cm) at 16 weeks, and I'm just curious.
 
#2 ·
I was told the earlier they're done the more accurate they are for dating purposes. My OB w/my first 2 always did one (only one during pg unless something strange came up) at the end of the 1st trimester b/c it was (in his opinion) the best balance of dating and development- all major organs are visible, etc. I knew the exact date w/dd- that u/s was the same +/- 3 days and I went in at 13 weeks. W/ds1, I postponed b/c I wanted to learn the sex and that u/s was the same +/- 7 days. W/ds2, I changed docs and did the u/s at 20 weeks, that was 10 days off of my edd, which I knew exactly when I conceived, so I went by that one. I had another one a few weeks later (changed docs again) and that was off about 2 weeks. So, the later I had an u/s, the further it was from my conception date.
 
#3 ·
The guidelines that should be used when determining gestational age by US are:
If LMP is known-
up to 19 week, 6 days +/- 7 days
up to 29 week, 6 days +/- 14 days
30 weeks or more +/- 21 days

So this means if at 12 weeks your US is within 7 days of agreement with your LMP, the LMPwith 28 day cycle should be used to determine the correct EDD.

Establish the EDD early in the pregnancy and don't change it. Need to adjust it plus or minus for longer or shorter cycles.

The problem with US is that when they are used to determine GA they are measuring bones. If your family has tall or short people, anything other than average, it will throw off the GA. This is also the reason that US should not be used to determine the baby's weight. It can't measure soft tissue like muscle and can easily be off up to 2 lbs. at term. If the US tech is already looking at a diagnosis of macrosomia, it would be easy enough to err that way with the US measurements.

Hope that helps.
 
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