How accurate are the weights from growth scans?

mamam · May 2010

I remember getting estimates with my singleton, but I thought there was a lot of room for error. Are the growth scans more accurate? I'll ask my U/S tech tomorrow when I go in, but curious about your experiences.

E&RMommy · May 2010

There is always a standard deviation. It just depends on how far along you are. At 22 weeks it is likely +/- 100-200 grams. At term, it can be +/- a pound. It also depends on the skill of the person doing the ultrasound.

sourpop · May 2010

I was curious as well and asked my u/s tech this same question. She said basically its just an idea based on measurements of the baby.

nygrl79 · May 2010

I was told about +/-15%, which ends up to be a lot in a newborn. I was also told that the growth scans are really most accurate before 30 weeks gestation and then become less so. It also depends on the position of the baby, how easy it is to measure him/her. Exactly one week before my c/s, I was told that twin A was 6lb8oz and twin B is 5lb 6oz and they were born at 6lb11oz and 5lb3oz, so the estimate was most accurate for twin A.

anna2006 · May 2010

There are dozens of formulae used to calculate estimated fetal weight using the parameters they measure at the growth scan. Each medical center picks the one they trust, and each formula has its own standard deviation. Ask your tech about the one that they use, but in general, at term it can be off by up to a pound. More important than the actual weight is the percentile measurement and whether or not there is a discrepancy between the two weights.

Temp159135562506975 · May 2010

my boys were pretty close to the estimates- but i know someone who was estimated to have a 7lb baby and her baby was 11 lbs - yes, ELEVEN. So sometimes they are very off!

KellyD01 · May 2010

I know a woman who was told she needed to have a c/s for a 10# at last scan baby (1 week later she delivered a 6#6oz baby!) I know many others who've had scans that have been really off... but this was the worst! 3rd trimester u/s are notoriously inaccurate!!!!

shellshavintwins · May 2010

We were told at all but one growth scan that both our daughters were within 2-4 oz of each other. One said there was a 2 lb difference, but at the next one (the only time they did the growth scans within one week of each other) they were exactly the same and were the rest of the pregnancy. The last one said they were both 6lb 8oz. My baby A was 6lb 3oz and 19 1/2 inches my Baby B was 4lb11oz and 18inches. I was pretty sure several times she was measuring the same baby twice (they looked different even in their ultrasounds) but never said anything! It all depends, but most important is that they are continuing to grow!

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