Natural Birth

Curiosity question re: growth u/s

Does anyone on here know what measurements they actually use to estimate weight?  I'm just curious how it works.  I know they're not super accurate, so I'm not putting too much stock in ours, I was just watching them measure this morning and looking at each of the measurements and percentiles, and my nerd brain got all turned on by the math, and then they came up with a totally different estimate from what I would have expected.

So it seemed like a lot of abracadabra, if you know what I mean....  Any input? 

Re: Curiosity question re: growth u/s

  • At our last u/s, they described it by saying "We measure the head size, femur length, abdomen size, [and one other I can't remember] and then the computer takes that and applies formulas and a little magic to give us a number we can put down but not actually base any expectations on."

    So I think you're right on with the little magic and lot of random.

    image
    Mother's Day, 2011
  • At our last u/s, they described it by saying "We measure the head size, femur length, abdomen size, [and one other I can't remember] and then the computer takes that and applies formulas and a little magic to give us a number we can put down but not actually base any expectations on."

    So I think you're right on with the little magic and lot of random.

    image
    Mother's Day, 2011
  • image triumphantreturnofplanningbug:

    At our last u/s, they described it by saying "We measure the head size, femur length, abdomen size, [and one other I can't remember] and then the computer takes that and applies formulas and a little magic to give us a number we can put down but not actually base any expectations on."

    So I think you're right on with the little magic and lot of random.

    as far as I knew it was just those three... but sometimes they're so wrong, it's crazy. When they were measuring CRL still (a long time ago it feels like) I remember seeing the sonographer measure the CRL and she wasn't even measuring a little bit in the right place. And then she told me we were more than a week behind. I asked her to double check because everything had been okay to this point, and sure enough, it was only like 3 days (which is fine).

    Just a tiny movement of the cursor and the measurement is off. It's SO subjective.

    So I totally agree on magic and random! Stick out tongue

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  • My suspicion is that the previous poster is right re: the importance of the measurements.  The size estimate is only going to be as accurate as the measurements that are put into the model.  Every measure has an element of error and from watching the ultrasound techs, it looks to me like there's a lot of room for error (of the statistical sort, not that the measurements are wrong, per se).  I'd be willing to bet that the measurements would vary widely depending on the person doing the measuring, their experience, their mood, etc. and on other factors like the position of the baby. They say that medicine is an art and a science for a reason (not that it's any comfort to us moms to be).

    I doubt that hospitals have any sort of "feedback mechanism" where they compare the actual baby to what was measured on the growth scans. That would be an interesting study.

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  • I don't know it it is what they do, but the computer could integrate the areas that they have taken pictures of and use that as an estimate.  I know it can get sufficient data to do it, just don't know if that's actually what it does.  Considering with a press of a button the tech can go from one cross section to the perpendicular, that shows it's receiving enough data to do the calculation. 
  • In general, I agree that most est. are wrong, but my perinatologist was spot on - DS was "weighed" 1 week and 2 days before birth at 6lb 5oz, born @ 6lb 14.4oz (left the hospital at 6-5). I was really impressed by it.
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  • I've had the tech explain it to me before, but it sort of goes in one ear and out the other.  She told me the most important thing to remember is that it can be off up to a pound either way.  My baby measured at 6 pounds which she translated to mean between 5 and 7 pounds.  
  • I had a growth u/s yesterday since my fundal height and weight haven't increased in a few weeks. The tech measured the head, abdomen and femur and gave me an estimate of the baby's current size of 6 pounds 10 ounces and a margin of error of +/- 7 ounces. She did say that she thought it was a little on the high side since DH and I were both little babies.
  • The PP's are right, they measured abdomen, head and femur length with us as well.

    I had a growth U/S with DD 2 days before she was born. They estimated her to be at 7lb 5oz and she was born at 6lb 13oz.  So there wasn't a huge difference there.

    With DS they did the growth u/s at 36weeks and said he was 5lbs 3oz then.  My fluid was low so the following u/s's only looked at my fluid levels. But he was born at 6lbs 12ozs so I think his growth in 4-5 weeks was pretty typical. 

     I have heard of some babies measuring way off though :)

  • I was just googling this today because we were told last week we're measuring a week behind.  I'm not getting too worked up, but was curious.  This site has some really great info:

    http://www.baby2see.com/medical/charts.html
  • I've read that each technician has their own favorite equation they use, so even with the same measurements, you might get different estimates.
  • They measure the head circumference, abdomen circumference, and femur length and put it into some computer formula. My midwife said it can be off by as much as 2 lbs - pretty huge difference for something that weighs under 8 lbs :)

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