How accurate the ultrasound weight estimate is
A number on a screen is not a measurement.
An ultrasound does not weigh your baby. It measures the head, abdomen and thigh bone, then applies a formula. The result carries meaningful error, and the error is largest for the biggest babies, which is exactly where the decision is being made.
In the largest review of accuracy available, covering 41 studies and 112,034 patients, an estimate above 4,000 grams did make a genuinely large baby considerably more likely. But it predicted shoulder dystocia badly, correctly flagging only 22% of the cases that actually occurred. The measurement is useful for describing a baby and poor for deciding an operation.
Question to ask"What is the margin of error on this estimate, and would the recommendation change if the true weight were 400 grams lower?"
Where this comes from:
270. Moraitis AA, Shreeve N, Sovio U, Brocklehurst P, Heazell AEP, Thornton JG, et al. Universal third-trimester ultrasonic screening using fetal macrosomia in the prediction of adverse perinatal outcome: a systematic review and meta-analysis of diagnostic test accuracy. PLoS Med. 2020;17(10):e1003190.
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This page is educational. It supports the conversation with your own clinicians. It is not consent to any treatment, and it cannot assess you. If you are worried about a symptom now, see urgent warning signs.