Estimated Foetal Weight (Hadlock)

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.

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.