Understanding Fetal Weight Percentiles

Estimated fetal weight (EFW) is an ultrasound estimate, not a direct weighing of the baby. Its percentile is interpreted against a specified growth reference and gestational age. SMFM recommends population-based references such as Hadlock for this comparison. SMFM fetal growth guidance

This page is a guide, not an active percentile calculator. The earlier calculation was withheld because its reference and method were not adequately established. Use the percentile supplied by your maternity team and ask which chart and gestational age were used. A rough table cannot reconstruct the full ultrasound assessment.

What Fetal Weight Percentile Means

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Key Takeaway: SMFM defines fetal growth restriction (FGR) as an ultrasound EFW or abdominal circumference below the 10th percentile for gestational age. Severe FGR includes EFW below the 3rd percentile. These are clinical criteria, not an instruction to diagnose yourself from a website. SMFM recommendations 1 and 10

Report information	What to clarify with your team
Estimated fetal weight	Its units and the ultrasound method
EFW percentile	The reference used and exact gestational age
Abdominal circumference percentile	Whether it raises a separate growth concern
EFW or AC below the 10th percentile	Whether the team has assessed FGR and planned follow-up
EFW below the 3rd percentile	The team's assessment of severe FGR

A value above those thresholds is not a guarantee that every aspect of pregnancy is healthy. Likewise, do not wait for a second low value before following your team's instructions. The prior labels “severely SGA” and “severely LGA” are not used here as automatic diagnoses from this tool.

For an arithmetic interpretation only, the median is the middle of a reference distribution. Being closer to the median is not a performance score or proof of better health. A percentile is meaningful only when its underlying measurement and reference are specified.

Estimated Fetal Weight by Week

The previous week-by-week gram table has been withheld: its entries could not be traced to one verified source table and method. Naming WHO and Hadlock together did not establish that the numbers came from either. Preserving unsupported grams would create false precision.

Instead, use this checklist to read an actual report:

Record alongside the estimate	Reason to retain it
Scan date	Identifies the actual examination
Gestational weeks and days	Avoids rounding a partial week into a different comparison
EFW in grams	Prevents a grams/kilograms transcription error
Reported percentile	Keeps the clinician's comparison rather than inventing another
Reference name	Identifies the chart used
Team's interpretation and next appointment	Keeps clinical advice separate from the numerical result

This checklist is a recordkeeping aid authored for this page, not a clinical scoring system. No new unverified weight table replaces the old one. Your maternity team can explain the chart used; the source supports the reference-selection principle, not an accuracy guarantee for an individual scan. SMFM reference recommendation

When Growth Percentile Is Concerning

SMFM's FGR guidance considers maternal, fetal and placental causes, and recommends further clinical evaluation and surveillance when FGR is diagnosed. Umbilical artery Doppler is one element of that monitoring. The calculator cannot choose a monitoring interval or delivery plan. SMFM assessment and surveillance recommendations

Changing percentiles: Bring previous reports to the team, but do not infer a specific placental cause from two percentiles alone. This guide does not declare that a consistently low value is harmless or that a falling value proves a diagnosis.

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Warning: The earlier fixed ±10–15% error range and assurance that earlier scans are always more accurate are withheld. This page has not verified a universal error interval for every method and population. Ask the team about the uncertainty of the specific estimate and follow the assessment they recommend.

The useful next step is to understand what was measured, how the percentile was obtained and what follow-up was advised. Changing calculator websites does not settle a clinical disagreement.

Frequently Asked Questions

What is a normal fetal weight percentile?

This guide does not use a 10th–90th band as a guarantee of health. SMFM uses EFW or AC below the 10th percentile as its FGR definition, with the full clinical assessment determining care. SMFM definition

Should I worry about a low percentile?

Discuss it promptly with your maternity team and follow their instructions. A low EFW or AC can meet the guideline criterion for FGR; do not assume that it is harmless because only one scan is available. The team evaluates its meaning and follow-up. SMFM guidance

How accurate are ultrasound weight estimates?

They remain estimates. This page assigns no universal percentage error or exact true-weight interval and does not calculate a percentile. Ask which method and reference were used and what uncertainty applies to your report. SMFM's recommendation to use a population reference does not validate a generic site's gram table. SMFM reference guidance

Sources

SMFM Consult Series #52: Diagnosis and management of fetal growth restriction, 2020, reaffirmed 2024; definitions, reference selection and clinical monitoring. Accessed October 5, 2026. This source-based guide is not practitioner review or an individualized clinical assessment.