Third-trimester estimated fetal weight carries a margin of error of roughly 10% either side, which at term is 300 to 400 grams. A note reading '3,950 g' invites a decision; a note reading '3,950 g, so plausibly 3,550 to 4,350 g' invites the right one.
The habit costs nothing and changes two conversations. It stops a single scan crossing an arbitrary threshold from becoming the reason for an induction or a caesarean, and it makes the counselling honest when a woman asks how sure you are. Where the estimate genuinely matters - suspected macrosomia in a diabetic pregnancy, a growth-restricted fetus near a delivery decision - pair it with what is actually informative: serial measurements, liquor volume, Dopplers, and the clinical picture.
Say the same thing out loud. Women remember a number, and they carry it into labour; they remember an uncertain range differently, and they make better decisions with it.
- Record estimated fetal weight with a plus or minus 10% range in the notes and the letter
- Never let a single estimate above a threshold be the sole indication for delivery
- Prefer serial growth plus Dopplers and liquor over one cross-sectional weight
- Tell the woman the range, not just the figure, when discussing mode of delivery
- Re-check the measurement if the estimate jumps implausibly between scans
Why it matters
Most of the harm from fetal weight estimation comes from acting on the precision of a number it does not have.
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