Pregnancy · Growth
Estimated Fetal Weight (Hadlock)
Estimate fetal weight from ultrasound biometry (Hadlock formula). Plus what accuracy you can actually expect (±10-15%), what SGA / macrosomia mean, when growth scans are offered, and how to interpret your results.
Last reviewed 31 May 2026
Hadlock IV estimated fetal weight
Estimate fetal weight from ultrasound measurements
Enter all four measurements from your latest ultrasound report (units in millimetres) to see the estimated fetal weight.
How is fetal weight estimated?
Ultrasound biometry — measurements of baby’s body parts on scan — fed into the Hadlock formula:
- Biparietal diameter (BPD) — head width
- Head circumference (HC)
- Abdominal circumference (AC) — most influential
- Femur length (FL) — thigh bone
Calculator uses Hadlock 4 (BPD + HC + AC + FL — most accurate variant). Original publication: Hadlock FP 1985 American J Obstet Gynecol.
How accurate is the EFW?
Modest. 95% confidence interval ±10-15%. A 3,000 g estimate could be 2,550-3,450 g actual. Accuracy depends on:
- Ultrasound technique.
- Baby’s position.
- Amniotic fluid volume.
- Maternal BMI.
- Gestational age (late T3 less accurate).
Best uses: detecting concerning growth restriction or macrosomia; not for predicting exact birth weight.
Why is my baby's growth being scanned?
Reasons for serial growth ultrasound:
- High-risk pregnancies (preeclampsia, hypertension, diabetes, GDM, lupus, multiples).
- Suspected SGA from fundal height or risk factors.
- Suspected LGA / macrosomia.
- Previous unexplained stillbirth.
- Reduced fetal movements.
- Previous growth-restricted baby.
- Advanced maternal age (35+) with other factors.
What is SGA / FGR?
- SGA (small for gestational age): baby measuring below 10th centile for gestational age.
- FGR (fetal growth restriction): SGA + abnormal Doppler / reduced growth velocity / oligohydramnios / abnormal CTG — the CONCERNING subset.
Some babies are constitutionally small (small parents, healthy growth on own curve). The TREND and DOPPLER matter more than single number.
What is macrosomia?
Baby measuring above 90th centile, or estimated weight > 4-4.5 kg.
- Risks: birth trauma (shoulder dystocia ~10% if EFW > 4.5 kg); caesarean rate higher; neonatal hypoglycaemia; prolonged labour.
- Causes: GDM / diabetes (most common); maternal obesity; previous macrosomic baby; post-term; ethnic background; idiopathic.
- Management: glucose control; consider planned caesarean if EFW > 4.5 kg per NICE; closer intrapartum monitoring.
What is Doppler ultrasound?
Specialised ultrasound assessing blood flow in fetal vessels:
- Umbilical artery — placental resistance.
- Middle cerebral artery (MCA) — fetal brain blood flow.
- Ductus venosus — fetal cardiac flow (late marker).
- Uterine artery — maternal side (often 20-22 weeks).
Abnormal Doppler suggests placental insufficiency. Used with growth biometry and amniotic fluid for FGR assessment.
What is the GROW chart?
Gestation-Related Optimal Weight chart — customised for maternal ethnicity, parity, height, weight. Each pregnancy gets its own customised centiles. Detects growth restriction better than uncustomised population charts. Used in many UK maternity units (Saving Babies’ Lives Care Bundle).
Different scenarios — what your scan means
Scenario 1: 32-week scan, EFW 1.8 kg = ~50th centile
Tracking normal range. Reassuring. No additional surveillance unless other concerns.
Scenario 2: 28-week scan, EFW 950 g = ~5th centile, parents small
Possibly constitutionally small. Check Doppler + amniotic fluid + previous growth curve. If reassuring, may track this centile. Repeat in 2-3 weeks.
Scenario 3: 34-week scan, EFW 3.0 kg = ~90th centile, GDM
Macrosomia trend. Optimise glucose control. Repeat scan at 36-37 weeks. Discuss delivery mode if EFW > 4.5 kg by 38-39 weeks.
Scenario 4: Centiles dropped from 50th at 28 wk to 8th at 32 wk
Concerning. FGR workup: Doppler, CTG, amniotic fluid. More frequent monitoring. Possible earlier delivery 34-37 weeks depending on severity.
Scenario 5: Asymmetric growth — small abdomen, normal head, 33 weeks
Late-onset FGR pattern (head-sparing). Placental insufficiency suspected. Doppler studies + repeat scan + CTG monitoring. Often delivered 36-37 weeks if Doppler abnormal.
How often will I have growth scans?
- Low-risk: anomaly scan at 18-22 weeks; no further routine growth scans.
- High-risk / growth concern: 28, 32, 36 weeks usually.
- FGR / significant concern: 2-weekly Dopplers; sometimes weekly CTGs after 30 weeks.
What are 3D / 4D scans?
For parents’ experience, not clinical. 3D = still-frame images; 4D = moving. PURPOSE: bonding, photos. CLINICAL growth scans use 2D ultrasound. Available privately (£100-300). No proven harm, no clinical benefit beyond standard 2D.
Limitations of EFW
- ±10-15% confidence interval — not exact.
- Late-T3 estimates can be off by 200-500 g.
- Higher maternal BMI reduces accuracy.
- Doesn’t replace clinical judgement.
- Trend matters more than single reading.
Sources
- 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 31. Investigation and management of the small-for-gestational-age fetus.
- NICE NHS Saving Babies’ Lives Care Bundle Version 3.
- Papageorghiou AT, et al. INTERGROWTH-21st fetal growth standards. Lancet 2014.
- WHO. Fetal growth charts.
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Frequently asked questions
How accurate is the estimated fetal weight (EFW)?
Modest. Standard ultrasound EFW (Hadlock formula) has 95% confidence interval of approximately ±10-15% — so a 3,000 g estimate could be anywhere from 2,550 to 3,450 g. Accuracy depends on: ultrasound technique, baby's position, amniotic fluid volume, maternal BMI (higher BMI = harder ultrasound), gestational age (later third-trimester less accurate). Best uses: detecting concerning growth restriction (low weight) or macrosomia (high weight); not for predicting birth weight to the gram.
What is the Hadlock formula?
Most widely used fetal weight estimation formula. Uses combinations of: BIPARIETAL DIAMETER (BPD — head width); HEAD CIRCUMFERENCE (HC); ABDOMINAL CIRCUMFERENCE (AC — most influential); FEMUR LENGTH (FL — thigh bone). Different Hadlock variants: HADLOCK 1 (BPD + AC), HADLOCK 2 (BPD + AC + FL), HADLOCK 3 (HC + AC + FL), HADLOCK 4 (BPD + HC + AC + FL — most accurate). Original publication: Hadlock FP 1985, American Journal of Obstetrics & Gynecology. Calculator uses Hadlock 4.
Why is my baby's growth being scanned?
Reasons for serial growth ultrasound in pregnancy: HIGH-RISK pregnancies (preeclampsia, hypertension, diabetes including GDM, lupus, multiple pregnancy); SUSPECTED SMALL-FOR-GESTATIONAL-AGE (SGA) baby from fundal height or maternal risk factors; SUSPECTED LARGE-FOR-GESTATIONAL-AGE (LGA / macrosomia) from fundal height or GDM; PREVIOUS unexplained stillbirth; REDUCED FETAL MOVEMENTS; previous growth-restricted baby; recurrent miscarriage; advanced maternal age (35+ for SGA, with other factors). NICE / RCOG protocols guide scan timing.
What is small-for-gestational-age (SGA)?
Baby measuring below 10th centile for gestational age — i.e. smaller than 90% of babies at that gestation. Doesn't always mean problem — some babies are constitutionally small (small parents, healthy growth on own curve). FGR (Fetal Growth Restriction) is the concerning subset: SGA + abnormal Doppler / reduced growth velocity / oligohydramnios / abnormal CTG. Causes: placental insufficiency (most common), congenital infection, chromosomal abnormality, maternal substance use, maternal severe undernutrition, multiple pregnancy. Management depends on cause and severity.
What is macrosomia?
Baby measuring above 90th centile, or estimated weight > 4-4.5 kg. Risks: birth trauma (shoulder dystocia, ~10% if EFW > 4.5 kg); caesarean rate higher; neonatal hypoglycaemia; prolonged labour. Causes: GDM / diabetes (most common); maternal obesity; previous macrosomic baby; post-term pregnancy; ethnic background; idiopathic. Management: glucose control if diabetic; consider planned caesarean if EFW > 4.5 kg per NICE; closer intrapartum monitoring.
Can ultrasound predict my baby's exact birth weight?
No. The ±10-15% error margin means EFW close to birth gives a range, not exact number. 3,200 g estimate could be 2,720-3,680 g actual. Late-third-trimester ultrasounds (37+ weeks) tend to OVER-estimate or UNDER-estimate by 200-500 g. Birth weight is the actual measurement. EFW is for growth-tracking and clinical decision-making, not predicting birthday weight.
What if my baby is measuring small?
Depends on context. SOMETIMES NORMAL: small parents, ethnic background; constitutionally small but growing on own curve. ASSESSMENT NEEDED: GROWTH CURVE (is it tracking centile or dropping?); DOPPLER (uterine and umbilical artery flow); AMNIOTIC FLUID; FETAL MOVEMENTS. If FGR confirmed: more frequent monitoring (weekly Doppler, CTG); possible earlier induction or caesarean depending on severity and gestational age. NICE / RCOG: monitor and deliver between 37-40 weeks usually.
What if my baby is measuring large?
Depends on cause and gestational age. WORKUP: rule out GDM if not already done (HbA1c, OGTT); ultrasound for confirmation of biometry; consider amniotic fluid (polyhydramnios common); review maternal weight gain. MANAGEMENT: optimise glycaemic control if diabetic; planned caesarean often considered for EFW > 4.5 kg per NICE; otherwise vaginal birth proceed with intrapartum monitoring; baby's blood glucose checked at birth especially if GDM. Shoulder dystocia risk discussion with team.
How often will I have growth scans?
Depends on risk. ROUTINE LOW-RISK pregnancy: anomaly scan at 18-22 weeks; no further routine growth scans. HIGH-RISK or growth concern: 28 weeks; 32 weeks; 36 weeks usually. FGR or significant concern: 2-weekly Dopplers; sometimes weekly CTGs after 30 weeks. NICE Saving Babies' Lives Care Bundle: standardised approach to growth surveillance for high-risk pregnancies.
What is Doppler ultrasound?
Specialised ultrasound assessing BLOOD FLOW in fetal vessels. Used to check placental function. ASSESSMENTS: UMBILICAL ARTERY (cord vessel — placental resistance); MIDDLE CEREBRAL ARTERY (MCA — fetal brain blood flow); DUCTUS VENOSUS (fetal cardiac flow — late marker of decompensation); UTERINE ARTERY (maternal side, often done at 20-22 weeks). Abnormal Doppler patterns suggest placental insufficiency. Used in combination with growth biometry and amniotic fluid for FGR / placental concerns.
What is fundal height and is it accurate?
Measurement from top of pubic bone to top of uterus, in cm. From 20 weeks onwards, fundal height in cm roughly equals gestational age in weeks (±2-3 cm normal range). Used as a SCREENING tool — if 3+ cm out from expected, growth scan recommended. Sensitivity 40-60% — misses some growth issues. Many maternity units now use customised charts (GROW chart — adjusts for ethnicity, parity, maternal characteristics). More reliable than uncustomised charts. NICE Saving Babies' Lives uses customised charts.
What is the GROW chart?
Gestation-Related Optimal Weight chart — customised growth chart adjusting for maternal ethnicity, parity, height, weight, and other factors that legitimately influence baby's size. Each pregnancy gets its own customised centiles. Detects growth restriction better than uncustomised population charts. Used in many UK maternity units. Plot fundal height and EFW on the GROW chart at each visit.
My baby is in the 5th centile — should I worry?
Depends on TREND and DOPPLER. If: tracking own curve consistently (always around 5th), placenta blood flow normal, fetal movements good, amniotic fluid normal — probably constitutionally small. If: dropping centiles (was 25th, now 5th), abnormal Doppler, reduced movements, or low amniotic fluid — concerning. Discussion with consultant. Possible serial monitoring + earlier delivery. Don't panic on single number — context matters.
Are 3D / 4D ultrasounds for growth?
No — they're for parents' experience, not clinical. 3D shows still-frame images; 4D shows moving images. PURPOSE: bonding, photos, gender reveal. CLINICAL GROWTH SCANS use 2D ultrasound for accurate biometry. Most NHS units don't routinely offer 3D/4D; available privately (£100-300). No proven harm but no clinical benefit beyond standard 2D. Some 4D ultrasound facilities also do clinical assessment (gender, position, fluid).
What if my baby's head is measuring big or small?
ISOLATED head measurement out of range is usually variation — head circumference family-pattern (mum/dad head size correlates). Worry if: HEAD significantly bigger than abdomen suggesting growth restriction with head-sparing (typical late-onset FGR pattern); HEAD very small or hydrocephalus suspected. Always interpreted in context of all measurements + Doppler + maternal history. Discuss with obstetric team.
What is asymmetric vs symmetric growth restriction?
ASYMMETRIC FGR: head growth preserved relative to body — suggests LATE-ONSET placental insufficiency (head-sparing effect, brain protection). More common. Typically presents in third trimester. SYMMETRIC FGR: all measurements equally small — suggests EARLY-ONSET cause (congenital infection, chromosomal abnormality, maternal substance use); often present from anomaly scan onwards. Different prognosis and management.
How does this relate to other calculators on BumpBites?
Companion: /calculators/baby-size-by-week for week-by-week median size; /calculators/baby-percentile for after-birth growth; /calculators/fgr-doppler-composite for FGR clinical scoring; /calculators/biophysical-profile (BPP); /calculators/afi-sdp for amniotic fluid; /calculators/mca-psv for middle cerebral artery Doppler.