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Estimated fetal weight (EFW) is an ultrasound-derived calculation that predicts the weight of a fetus in utero by combining measurements of fetal biometry. Accurate EFW is central to the surveillance and management of fetal growth disorders — both intrauterine growth restriction (IUGR), where the fetus fails to reach its growth potential, and macrosomia, where the fetus is abnormally large. EFW is determined by applying mathematical formulae to ultrasound measurements of the biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). The most widely used formula is the Hadlock 4-parameter model: log10(EFW) = 1.3596 + 0.0064×HC + 0.0424×AC + 0.174×FL + 0.00061×BPD×AC − 0.00386×AC×FL. EFW is then plotted on a customised or population-based growth chart appropriate for gestational age, sex, ethnicity, and maternal characteristics. A fetus whose EFW falls below the 10th centile for gestational age is classified as small for gestational age (SGA); below the 3rd centile is considered severe SGA and correlates more closely with true growth restriction. Growth velocity — the change in EFW over time — is as important as the absolute value: a fetus crossing centiles downward (e.g., moving from the 30th to 8th centile over 4 weeks) represents pathological growth restriction even if it remains above the 10th centile. EFW has a measurement error of approximately ±10-15%, which must be factored into clinical decisions, particularly near important thresholds such as 500 g (viability) or 1500 g (key prematurity milestone).
Hadlock 4-parameter: log10(EFW) = 1.3596 + (0.0064×HC) + (0.0424×AC) + (0.174×FL) + (0.00061×BPD×AC) − (0.00386×AC×FL); EFW in grams; AC most heavily weighted; SGA = EFW <10th centile; Severe SGA <3rd centile
- 1Perform a detailed obstetric ultrasound with the fetus in an appropriate position. Measure the biparietal diameter (BPD) in the transventricular plane at the widest cranial diameter.
- 2Measure the head circumference (HC) in the same transventricular plane using an ellipse trace around the outer calvarial margin.
- 3Measure the abdominal circumference (AC) in a transverse plane at the level of the fetal stomach and umbilical vein insertion into the liver — the AC is the single most important measurement for detecting growth restriction.
- 4Measure the femur length (FL) with the femur horizontal and both ends of the ossified diaphysis visible, measuring from end to end of the diaphysis (excluding the distal femoral epiphysis).
- 5Apply the Hadlock formula (or equivalent validated formula in the local clinical system) to calculate EFW in grams. Most ultrasound machines calculate this automatically.
- 6Plot EFW on a gestational age-specific growth chart. For UK practice, the GROW (Gestation Related Optimal Weight) customised chart adjusts for maternal height, weight, ethnicity, and parity. Identify the centile and note the trend from previous measurements.
- 7Assess EFW in context: a single below-the-10th-centile value requires further evaluation (Doppler studies of umbilical artery, middle cerebral artery, and ductus venosus) to determine whether the fetus is constitutionally small or truly growth-restricted with placental insufficiency.
Continue routine surveillance; next growth scan at 36 weeks if uncomplicated
EFW at the 50th centile for gestational age with symmetric biometry indicates normal fetal growth. No further intervention is needed beyond routine antenatal care.
Umbilical artery Doppler essential; consider corticosteroids; timing of delivery planning
Centile crossing from 35th to 5th over 4 weeks indicates pathological growth restriction, not constitutional smallness. Umbilical artery Doppler is the key surveillance tool for distinguishing healthy small from at-risk growth-restricted fetuses.
Discuss mode of delivery; elective caesarean if EFW >4.5 kg or severe macrosomia in diabetic pregnancy
In diabetic pregnancies, fetal macrosomia is concentrated in the trunk (AC enlargement) due to insulin-driven hepatic glycogen deposition and subcutaneous fat. HC/AC ratio is useful for detecting this pattern.
Ductus venosus Doppler and CTG required; delivery likely 30-32 weeks to prevent stillbirth
Absent end-diastolic flow in the umbilical artery indicates severely compromised placental circulation. This is an emergency finding requiring immediate specialist review and planning for delivery, balancing stillbirth risk against prematurity risks.
Primary care physicians and internists use Fetal Weight Estimate during routine clinical assessments to screen patients, establish baselines for longitudinal monitoring, and identify individuals who may need referral to specialists for further diagnostic evaluation or therapeutic intervention.
Hospital clinical pharmacists apply Fetal Weight Estimate to verify drug dosing calculations, particularly for medications with narrow therapeutic indices like warfarin, aminoglycosides, and chemotherapy agents where patient-specific factors such as renal function and body weight critically affect safe dosing ranges.
Public health epidemiologists use Fetal Weight Estimate in population-level screening programs to calculate disease prevalence, assess screening test sensitivity and specificity, and determine the number needed to screen to detect one case in various demographic subgroups.
Clinical researchers incorporate Fetal Weight Estimate into study design protocols to calculate sample sizes, determine statistical power for detecting clinically meaningful differences, and establish inclusion criteria based on quantitative physiological thresholds.
Pediatric versus adult reference ranges
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in fetal weight estimate calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
Pregnancy and hormonal variations
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in fetal weight estimate calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
Extreme body composition
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in fetal weight estimate calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
Multiple Pregnancy
Each twin is assessed independently for EFW. Discordance — defined as a difference in EFW between twins of ≥20% — is a significant finding, particularly in monochorionic pregnancies where it may indicate early twin-to-twin transfusion syndrome. Growth surveillance frequency is increased in multiple pregnancy compared to singleton.
| EFW Centile | Classification | Doppler Indicated | Action |
|---|---|---|---|
| ≥10th | Appropriate for GA | No (routine) | Routine monitoring |
| 3rd-9th | SGA | Yes | Serial growth scans + Doppler every 2 weeks |
| <3rd | Severe SGA | Yes urgently | Weekly Doppler; consider admission/delivery |
| Absent AEDF | Severe IUGR | Yes — emergency | Corticosteroids; deliver 30-34 wks |
| Reversed AEDF | Critical IUGR | Yes — critical | Deliver immediately or within 24-48 hours |
| >90th | Large for GA / Macrosomia | No (unless other concerns) | Assess for GDM; delivery planning |
How accurate is ultrasound EFW?
EFW has an inherent measurement error of approximately ±10-15% (one standard deviation). This means a calculated EFW of 2000g could represent an actual weight of approximately 1700-2300g. Accuracy is better in the mid-trimester and reduces in the third trimester, particularly for large or macrosomic fetuses. This uncertainty must be considered when making clinical decisions near critical weight thresholds.
Which formula is most accurate for EFW?
In the context of Fetal Weight Estimate, this depends on the specific inputs, assumptions, and goals of the user. The underlying formula provides a deterministic relationship between inputs and output, but real-world application requires interpreting the result within the broader context of health and medical practice. Professionals typically cross-reference calculator output with industry benchmarks, historical data, and regulatory requirements. For the most reliable results, ensure inputs are sourced from verified data, understand which assumptions the formula makes, and consider running multiple scenarios to bracket the range of likely outcomes.
What is customised fetal growth assessment?
Customised growth assessment (GROW method) adjusts the expected EFW for each individual pregnancy based on maternal height, weight, ethnicity, parity, and gestational age, generating a term optimal weight (TOW) and centile lines specific to that pregnancy. This reduces false-positive rates for SGA in smaller women and false-negative rates in larger women compared to population-based charts.
What is the difference between SGA and IUGR?
SGA (small for gestational age) is a statistical definition — fetal weight below a centile threshold (usually 10th). IUGR (intrauterine growth restriction) implies the fetus has failed to reach its genetic growth potential due to placental insufficiency. Not all SGA fetuses are growth-restricted (some are constitutionally small and healthy); not all growth-restricted fetuses are SGA (some may be at the 15th centile but have crossed from the 70th centile).
What is the role of umbilical artery Doppler in growth assessment?
Umbilical artery Doppler measures the resistance to blood flow through the placenta. An elevated pulsatility index (PI) or reduced/absent/reversed end-diastolic flow indicates increasing placental resistance, suggesting placental insufficiency. The severity of Doppler abnormality guides the urgency of delivery: absent end-diastolic flow is an emergency; reversed end-diastolic flow often mandates immediate delivery.
What growth velocity is considered normal?
Fetal weight approximately doubles between 28 and 36 weeks, gaining approximately 200-250g per week in the third trimester. A growth velocity below the 10th percentile for gestational age (approximately <150g/week at 32-36 weeks) is concerning for growth restriction. Interval growth scans are typically spaced 2-4 weeks apart to detect velocity changes.
At what EFW is a fetus considered potentially viable?
The threshold of viability is gestational-age-dependent rather than purely weight-based. At 23+0 to 24+6 weeks with a weight appropriate for gestational age, resuscitation is offered in most high-resource settings with appropriate parental counselling. EFW of approximately 500g at 23 weeks correlates with the biological minimum for survival, though survival rates are below 50% at this gestation even with full intensive care.
How does macrosomia affect delivery planning?
An EFW above 4.0-4.5 kg increases the risk of shoulder dystocia, perineal trauma, and operative delivery. In non-diabetic pregnancies, elective caesarean is generally not recommended until EFW exceeds 5.0 kg due to the ±15% measurement error (actual weight may be 4.25 kg). In diabetic pregnancies, the threshold for planned caesarean is lower (EFW ≥4.5 kg) as diabetic macrosomia is truncally distributed and shoulder dystocia risk is greater.
Pro Tip
The abdominal circumference (AC) is the single most sensitive biometric parameter for detecting fetal growth restriction. A persistently low or falling AC, even in the presence of a relatively normal HC, should prompt Doppler evaluation and clinical review. Always compare to the previous AC measurement and note the interval growth, not just the centile.
Did you know?
The Hadlock formula was derived by Dr Frank Hadlock and colleagues at the University of Texas Health Science Center, Houston, and published in 1985 based on a study of over 1,000 fetuses. Despite being derived from a racially specific (predominantly White American) population in the mid-1980s, it remains the global standard for EFW calculation nearly four decades later — a remarkable testament to the robustness of the underlying biometric relationships.
References
- ›Hadlock FP et al. Estimation of fetal weight with the use of head, body and femur measurements. Am J Obstet Gynecol 1985.
- ›RCOG Green-top Guideline 31 — Small for Gestational Age Fetus. 2013 (updated 2023).
- ›Gordijn SJ et al. Consensus definition of fetal growth restriction: a Delphi procedure. Ultrasound Obstet Gynecol 2016.
- ›Lees CC et al. ISUOG Practice Guidelines: diagnosis and management of small-for-gestational-age fetus and fetal growth restriction. Ultrasound Obstet Gynecol 2022.
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