Accuracy of fetal weight estimates (ultrasound)

StacieJane · May 2011

Anyone have any stats on this? I did a quick Google search and found tons of anecdotal evidence suggesting that the estimates are usually pretty off, but can't find any actual research. A friend is being told by her doctor that they need to induce at 38 weeks because her baby is already "so big" (supposedly 8 lbs at 36wk appt). She asked what I thought, and I told her, but wanted to give her something more concrete.

InfamousQBert · May 2011

i found this with "fetal weight ultrasound study"

https://www.nzma.org.nz/journal/119-1241/2146/

"Conclusion ... For one in four women, however, the fetal weight estimation was more than 10% different from the actual birth weight of their infant. Ultrasound measurements had a tendency to overestimate the weight of small infants while underestimating the weight of both large infants and the infants of diabetic mothers. As the reliability of ultrasound estimation of fetal weight to detect larger babies was poor, the use of such an objective measurement in the management of suspected macrosomia in term singleton pregnancies should be avoided.

InfamousQBert · May 2011

also, this one doesn't give the same info, but did find that overestimation of fetal weight led to higher incidences of cesarean intervention.

https://www.ncbi.nlm.nih.gov/pubmed/19254597

InfamousQBert · May 2011

sorry, one more. found this via a friend's blog about medically unnecessary inductions.

https://www.ajog.org/article/S0002-9378(08)00775-8/abstract

https://momstinfoilhat.wordpress.com/2009/12/13/nulliparous-psychosocial-induction/

thethomps · May 2011

No stats, but they were way off in DD's estimate. At 36 weeks they said she was 7.5 lbs. and she was born at 41 weeks and weighed 8 lbs. 4 oz.

ckolak · May 2011

They aren't terribly reliable. DD was estimated at 6lb 1oz at 36 weeks, and when she was born 4 weeks later she was only 6lb 7oz. Based on the u/s and estimated weight gain, she "should" have been 7.5-8lb.

KrisM86 · May 2011

Ultrasound weight estimations at the end of pregnancy are often up to 2 lbs off in either direction. Palpitation can be more accurate, when done correctly.

StacieJane · May 2011

InfamousQBert:
sorry, one more. found this via a friend's blog about medically unnecessary inductions.

https://www.ajog.org/article/S0002-9378(08)00775-8/abstract

https://momstinfoilhat.wordpress.com/2009/12/13/nulliparous-psychosocial-induction/

Thank you for all the AWESOME links! xoxo!

StacieJane · May 2011

Thanks, mamas, for sharing your experiences!

sweetfern · May 2011

I have no stats, but from personal experience, I don't take much stock in u/s weight estimates. DD was estimated to be 8 lbs 14 oz, when she was born the next evening, she weighed in at 7 lbs 14 oz. After an induced labor (2 weeks post term), no less, so I'm sure there was some fluid weight to that.

flyer23 · May 2011

StacieJane:
Anyone have any stats on this? I did a quick Google search and found tons of anecdotal evidence suggesting that the estimates are usually pretty off, but can't find any actual research. A friend is being told by her doctor that they need to induce at 38 weeks because her baby is already "so big" (supposedly 8 lbs at 36wk appt). She asked what I thought, and I told her, but wanted to give her something more concrete.

https://www.aafp.org/afp/2001/0701/p169.html

It's a summary of ACOG's guidelines on fetal macrosomia (aka a baby who is "so big"). A couple of choice tidbits, emphasis mine:

"Nonetheless, the results of these reports, along with published cost-effectiveness data, do not support prophylactic cesarean delivery for suspected fetal macrosomia with estimated weights of less than 5,000 g (11 lb), although some authors agree that cesarean delivery in these situations should be considered."

"In cases of term patients with suspected fetal macrosomia, current evidence does not support early induction of labor."

So... ACOG does not recommend induction for suspected fetal macrosomia AT ALL, PERIOD, END OF STORY. And they miiiiiiiiiiiiiiiiiiiiiiiiiight posssssssssssssssibly recommend a c-section for your friend if her baby gains THREE more pounds.

That should be enough right there to fight the induction, even if the ultrasound is completely accurate -- or, heck, even if the ultrasound is a gross underestimation. But since your question was actually around the accuracy of ultrasound for weight estimates, here's a more direct answer:

https://www.aafp.org/afp/2001/0115/p302.html

"the typical mean error [for ultrasonography] ranges from 300 to 550 g (11.6 to 19.4 oz)."

StacieJane · May 2011

flyer23:
StacieJane:
Anyone have any stats on this? I did a quick Google search and found tons of anecdotal evidence suggesting that the estimates are usually pretty off, but can't find any actual research. A friend is being told by her doctor that they need to induce at 38 weeks because her baby is already "so big" (supposedly 8 lbs at 36wk appt). She asked what I thought, and I told her, but wanted to give her something more concrete.

https://www.aafp.org/afp/2001/0701/p169.html

It's a summary of ACOG's guidelines on fetal macrosomia (aka a baby who is "so big"). A couple of choice tidbits, emphasis mine:

"Nonetheless, the results of these reports, along with published cost-effectiveness data, do not support prophylactic cesarean delivery for suspected fetal macrosomia with estimated weights of less than 5,000 g (11 lb), although some authors agree that cesarean delivery in these situations should be considered."

"In cases of term patients with suspected fetal macrosomia, current evidence does not support early induction of labor."

So... ACOG does not recommend induction for suspected fetal macrosomia AT ALL, PERIOD, END OF STORY. And they miiiiiiiiiiiiiiiiiiiiiiiiiight posssssssssssssssibly recommend a c-section for your friend if her baby gains THREE more pounds.

That should be enough right there to fight the induction, even if the ultrasound is completely accurate -- or, heck, even if the ultrasound is a gross underestimation. But since your question was actually around the accuracy of ultrasound for weight estimates, here's a more direct answer:

https://www.aafp.org/afp/2001/0115/p302.html

"the typical mean error [for ultrasonography] ranges from 300 to 550 g (11.6 to 19.4 oz)."

Wow, this is great info, thank you.

Accuracy of fetal weight estimates (ultrasound)

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