familia

Fetal Weight by Week (Percentile)

Q: What is the difference between estimated fetal weight (EFW) and actual birth weight?

Estimated fetal weight (EFW) from ultrasound is typically within ±15% of actual birth weight at delivery. A 2,000 g (4.4 lb) EFW may result in 1,700–2,300 g actual weight. Variation depends on ultrasound timing, biometric accuracy, and genuine biological differences. If EFW is <10th percentile, actual birth weight may still be normal; serial ultrasounds better predict true SGA.

Q: Why do I see different weight estimates on different ultrasounds?

Different sonographers, machine software versions, and measurement techniques (HC, AC, FL calipers) introduce ±50–100 g variation. Additionally, fetal weight naturally fluctuates based on hydration and placental perfusion. Trend matters more than single estimate: consistent growth across weeks indicates normal trajectory.

Q: Does <10th percentile mean my baby is sick?

No. <10th percentile (SGA) is a statistical descriptor, not a diagnosis. Some babies are constitutionally small (genetic/ethnic background). However, SGA warrants investigation for **placental insufficiency** (poor nutrient transfer), maternal hypertension, or infection. Your OB will order additional tests: Doppler ultrasound, non-stress test (NST), and delivery timing decisions. Most SGA babies deliver healthy.

Q: Is >90th percentile (LGA) a problem?

LGA increases risk of **shoulder dystocia** (shoulder stuck during vaginal delivery), **cesarean delivery**, and neonatal complications (hypoglycemia, jaundice). Maternal diabetes is the strongest LGA risk factor. If LGA is confirmed, your OB may recommend a diabetic screen, delivery timing discussion, and family delivery plan. Most LGA babies do well with appropriate management.

Q: When should I be concerned about slow growth?

Concern arises when **estimated weight percentile drops across ultrasounds** (e.g., 50th → 30th → 15th) or stays persistently <10th with abnormal Doppler. A single ultrasound <10th percentile is less alarming; serial measurements revealing **declining trajectory** suggest placental insufficiency. Discuss with your OB; additional testing (Doppler, NST, maternal labs) guides next steps.

Q: How accurate is this calculator?

Hadlock formula accuracy is ±10–15% in clinical practice, meaning a 2,000 g estimate ± 200–300 g. Accuracy is best **weeks 20–35**; degrades after 35 weeks and before 20 weeks. Accuracy depends on ultrasound biometric quality; poor HC/AC/FL measurements worsen estimates. This calculator applies published regression curves; your sonographer's actual measurements may differ.

Q: Does fetal weight impact delivery method (vaginal vs. cesarean)?

Yes, in specific scenarios. **Estimated LGA (>90th, especially >4,500 g) in a mother with diabetes** increases cesarean risk due to shoulder dystocia fear. **Estimated SGA (<10th) with abnormal Doppler** may warrant earlier delivery timing (induction or cesarean at 37–39 weeks) rather than expectant management. Actual birth weight, maternal pelvic size, and fetal position also determine final delivery mode.

Q: Can I calculate fetal weight before 20 weeks or after 40 weeks?

Hadlock formula is validated **20–40 weeks only**. Before 20 weeks (first trimester/early second trimester), use **crown-rump length (CRL)** charts instead. After 40 weeks, formula accuracy drops sharply; clinical estimates become unreliable. If you enter 40 weeks, this calculator will flag a warning. Consult your ultrasound report for appropriate charts by trimester.

Q: What factors affect fetal weight besides genetics?

**Maternal nutrition** (inadequate calories/protein → SGA), **maternal medical conditions** (diabetes → LGA; hypertension/preeclampsia → SGA), **placental function** (insufficiency → SGA), **intrauterine infections** (TORCH, COVID-19 → SGA), **maternal smoking/substance use** (→ SGA), and **fetal abnormalities** (genetic syndromes, cardiac defects → variable). Your OB assesses these during pregnancy.

Calculate estimated fetal weight by gestational week using Hadlock formula. Compare percentiles (10th–90th) to assess growth and identify small/large for gestational age.

Radiology data · June 2026
Edited by Martín Rodríguez
Private — runs on your device

Suggest an improvement

Calculator Free · Private

Data updated: Apr 28, 2026 · Source: Hadlock FP, Harrist RB, Carpenter RJ, et al. Sonographic estimation of fetal weight. The value of the femur length in addition to head and abdomen measurements. Radiology. 1984;150(2):535–540.

Edited by: Martín Rodríguez (editorial policy )

Suggest an improvement

🧮 Keep calculating

Pregnancy Weeks by Ultrasound (CRL)Calculate gestational age from crown-rump length ultrasound measurement. Compare with LMP dating and assess accuracy using Hadlock formula 2026.Percentile Calculator — Find Where Any Value RanksEnter your dataset and a value to instantly find its percentile rank, quartile, and Q1/Q2/Q3 thresholds. Works for exam scores, test results, health measurements, and any numeric data.Pregnancy Calendar Week-by-WeekCalculate your pregnancy timeline week-by-week with baby size milestones, trimester boundaries, and key prenatal screening appointments from LMP or due date.Pregnancy Weight GainCalculate expected weight gain during pregnancy by pre-pregnancy BMI. Based on IOM 2009 guidelines. Free calculator.

Have a website? Embed this calculator for free Free — copy the code and paste it on your website Embed on your site

<iframe src="https://hacecuentas.com/embed/fetal-weight-percentile-by-week" width="100%" height="560" style="border:1px solid #e2e8f0;border-radius:12px;max-width:720px" loading="lazy" title="Fetal Weight by Week (Percentile)"></iframe>
<p style="font-size:13px;text-align:center;margin:8px 0">Powered by <a href="https://hacecuentas.com" target="_blank" rel="noopener sponsored">Hacé Cuentas</a> — <a href="https://hacecuentas.com/fetal-weight-percentile-by-week" target="_blank" rel="noopener sponsored">Fetal Weight by Week (Percentile)</a></p>

Preview →

Paste it on your site. Keep the credit link — thanks for sharing. More widgets →

Want to change something?Edit any field and calculate again.

Instant resultIt recalculates in your browser, no page reload.

Fast and transparent

How to use this calculator

The main result first. The explanation you need, right after.

Step by step

Enter your data

Tap the Calculate button

Check the result

Estimate fetal weight during pregnancy using the Hadlock formula, the clinical standard for ultrasound-based weight prediction. Enter your gestational age (weeks) and desired percentile (10th, 25th, 50th, 75th, 90th) to get an accurate weight range in grams and pounds. This calculator helps identify whether your baby is tracking normally, small for gestational age (SGA), or large for gestational age (LGA).

When to use this calculator

Monitor fetal growth between prenatal ultrasounds
Compare baby's weight to standard growth curves
Identify small or large for gestational age (SGA/LGA)
Plan delivery timing if growth concerns exist
Track longitudinal weight gain week-to-week
Validate ultrasound estimates against clinical standards

Fetal Weight Reference by Gestational Week and Percentile (Hadlock)

Gestational Week	10th Percentile (g)	25th Percentile (g)	50th Percentile (g)	75th Percentile (g)	90th Percentile (g)
20	249	275	331	382	430
22	350	395	478	558	630
24	498	567	674	791	899
26	695	794	960	1,130	1,285
28	950	1,085	1,300	1,535	1,745
30	1,250	1,420	1,700	2,000	2,265
32	1,400	1,590	1,900	2,250	2,545
34	1,800	2,050	2,450	2,900	3,250
36	2,200	2,500	2,950	3,450	3,850
38	2,600	2,950	3,400	3,900	4,300
40	2,900	3,200	3,600	4,100	4,500

Fuente: Hadlock FP et al., Radiology 1984 / ACOG Practice Bulletin No. 204 (2019). Values are approximations from Hadlock's 1985 regression cohort (2,256 fetuses, 19–43 weeks). Margin of error ±15%. Not for diagnostic use; confirm with serial ultrasounds and clinical assessment.

How it works

What is the Hadlock formula?

The Hadlock formula is a clinical ultrasound-based method that estimates fetal weight during pregnancy using biometric measurements like head circumference, abdominal circumference, and femur length. It provides predictions accurate within ±15% between weeks 20 and 40 of gestation. Percentile comparison reveals whether growth falls within normal range or indicates small or large for gestational age conditions.

How Hadlock Formula Works

The Hadlock formula combines ultrasound biometric measurements to predict fetal weight:

Log10(Weight in grams) = 1.3596 + (0.0064 × HC) + (0.0424 × AC) + (0.174 × FL) − (0.00139 × AC × FL)

Where:

HC = Head Circumference (mm)

AC = Abdominal Circumference (mm)

FL = Femur Length (mm)

This calculator uses population regression curves by percentile derived from Hadlock's 1985 cohort study (2,256 fetuses, 19–43 weeks gestation).

Percentile Interpretation

Percentile	Growth Status	Clinical Meaning
<10th	Small for gestational age (SGA)	Below 10% of babies at this week; increased perinatal risk
10th–25th	Low-normal	Below average but not SGA; monitor growth
25th–75th	Normal range	Expected weight for week; typical fetal growth
75th–90th	High-normal	Above average but not LGA; monitor growth
>90th	Large for gestational age (LGA)	Above 90% of babies at this week; risk for complications

Worked Example

Scenario: Gestational age 32 weeks, 50th percentile (median baby)

1. Hadlock formula applied to week-32 data
2. Result: ~1,700 g (3.7 lb) at median
3. Normal range at 32 weeks: 25th = 1,400 g; 75th = 2,000 g
4. Interpretation: Baby's weight is on track for typical growth

When NOT to Apply / Limitations

Before 20 weeks or after 40 weeks: Formula accuracy decreases; use alternate charts (Lubchenco for <20 weeks)

Maternal conditions: Diabetes, hypertension, or placental insufficiency may affect actual weight vs. estimate

Genetic factors: Ethnic background and parental height influence normal birth weight ranges

Accuracy margin: ±15% at best; ultrasound factors (operator, machine) introduce variability

Not a diagnosis tool: SGA/LGA confirmation requires serial ultrasounds and clinical assessment, not a single calculation

Biometric input errors: If HC, AC, FL measurements from ultrasound are inaccurate, estimate is unreliable

Hadlock vs. Other Formulas

Hadlock: Most widely used in obstetrics; validated 1985–present; accuracy ±10–15% weeks 20–40

Shepard: Older (1982); slightly lower accuracy after 35 weeks

Combs & Gabbe: Emphasizes abdominal circumference; good for detecting growth restriction

Custom population curves: Account for ethnicity/genetics; used in specialized centers

This calculator uses Hadlock's regression coefficients as the standard of care in prenatal imaging.

Example: Median baby at 32 weeks

Inputs: gestational age 32 weeks, 50th percentile (median).

Hadlock estimate: the regression curve returns about 2,190 g at the median for week 32.

Pounds: 2,190 g ≈ 4.83 lb.

The normal 25th–75th range at this week spans roughly 1,930 g to 2,540 g, so this weight is right on track.

Estimated fetal weight is 2,190 g (4.83 lb). Reference prices, adjust as needed.

Disclaimer: Results are indicative and do not replace personalized guidance. Before making decisions that matter, consult a licensed professional in the relevant field.

Frequently asked questions

What is the difference between estimated fetal weight (EFW) and actual birth weight?

Estimated fetal weight (EFW) from ultrasound is typically within ±15% of actual birth weight at delivery. A 2,000 g (4.4 lb) EFW may result in 1,700–2,300 g actual weight. Variation depends on ultrasound timing, biometric accuracy, and genuine biological differences. If EFW is <10th percentile, actual birth weight may still be normal; serial ultrasounds better predict true SGA.

Why do I see different weight estimates on different ultrasounds?

Different sonographers, machine software versions, and measurement techniques (HC, AC, FL calipers) introduce ±50–100 g variation. Additionally, fetal weight naturally fluctuates based on hydration and placental perfusion. Trend matters more than single estimate: consistent growth across weeks indicates normal trajectory.

Does <10th percentile mean my baby is sick?

No. <10th percentile (SGA) is a statistical descriptor, not a diagnosis. Some babies are constitutionally small (genetic/ethnic background). However, SGA warrants investigation for placental insufficiency (poor nutrient transfer), maternal hypertension, or infection. Your OB will order additional tests: Doppler ultrasound, non-stress test (NST), and delivery timing decisions. Most SGA babies deliver healthy.

Is >90th percentile (LGA) a problem?

LGA increases risk of shoulder dystocia (shoulder stuck during vaginal delivery), cesarean delivery, and neonatal complications (hypoglycemia, jaundice). Maternal diabetes is the strongest LGA risk factor. If LGA is confirmed, your OB may recommend a diabetic screen, delivery timing discussion, and family delivery plan. Most LGA babies do well with appropriate management.

When should I be concerned about slow growth?

Concern arises when estimated weight percentile drops across ultrasounds (e.g., 50th → 30th → 15th) or stays persistently <10th with abnormal Doppler. A single ultrasound <10th percentile is less alarming; serial measurements revealing declining trajectory suggest placental insufficiency. Discuss with your OB; additional testing (Doppler, NST, maternal labs) guides next steps.

How accurate is this calculator?

Hadlock formula accuracy is ±10–15% in clinical practice, meaning a 2,000 g estimate ± 200–300 g. Accuracy is best weeks 20–35; degrades after 35 weeks and before 20 weeks. Accuracy depends on ultrasound biometric quality; poor HC/AC/FL measurements worsen estimates. This calculator applies published regression curves; your sonographer's actual measurements may differ.

Does fetal weight impact delivery method (vaginal vs. cesarean)?

Yes, in specific scenarios. Estimated LGA (>90th, especially >4,500 g) in a mother with diabetes increases cesarean risk due to shoulder dystocia fear. Estimated SGA (<10th) with abnormal Doppler may warrant earlier delivery timing (induction or cesarean at 37–39 weeks) rather than expectant management. Actual birth weight, maternal pelvic size, and fetal position also determine final delivery mode.

Can I calculate fetal weight before 20 weeks or after 40 weeks?

Hadlock formula is validated 20–40 weeks only. Before 20 weeks (first trimester/early second trimester), use crown-rump length (CRL) charts instead. After 40 weeks, formula accuracy drops sharply; clinical estimates become unreliable. If you enter <20 or >40 weeks, this calculator will flag a warning. Consult your ultrasound report for appropriate charts by trimester.

What factors affect fetal weight besides genetics?

Maternal nutrition (inadequate calories/protein → SGA), maternal medical conditions (diabetes → LGA; hypertension/preeclampsia → SGA), placental function (insufficiency → SGA), intrauterine infections (TORCH, COVID-19 → SGA), maternal smoking/substance use (→ SGA), and fetal abnormalities (genetic syndromes, cardiac defects → variable). Your OB assesses these during pregnancy.

How do I know if my baby's percentile is normal for my ethnic background?

Hadlock's original cohort (1985) was predominantly white, European ancestry. Some ethnic groups (e.g., Southeast Asian, South Asian) have lower average birth weights even when healthy. INTERGROWTH-21st (WHO) and NPEC (Canadian) curves incorporate ethnic adjustment. If your OB suspects ethnic adjustment is needed, they may reference alternative curves. For this calculator, Hadlock (standard global obstetric reference) applies; discuss ethnic-specific curves with your provider.

Sources & references

Hadlock FP, Harrist RB, Carpenter RJ, et al. Sonographic estimation of fetal weight. The value of the femur length in addition to head and abdomen measurements. Radiology. 1984;150(2):535–540. — Radiology (peer-reviewed journal via PubMed Central) (1984)
American College of Obstetricians and Gynecologists (ACOG). Intrauterine Growth Restriction. Practice Bulletin No. 204. Obstet Gynecol. 2019;133(2):e97–e109. — Obstetrics & Gynecology (ACOG official journal) (2019)
Villar J, Cheikh Ismail L, Victora CG, et al. INTERGROWTH-21st Consortium. International standards for newborn weight, length, and head circumference by gestational age and sex: the Newborn Cross-Sectional Study of the INTERGROWTH-21st Project. Lancet. 2014;384(9946):857–868. — The Lancet (peer-reviewed) (2014)
Lubchenco LO, Hansman C, Dressler M, et al. Intrauterine growth as estimated from liveborn birth-weight data at 24 to 42 weeks of gestation. Pediatrics. 1963;32(5):793–800. — Pediatrics (American Academy of Pediatrics) (1963)
Society for Maternal–Fetal Medicine. Screening and diagnosis of gestational diabetes mellitus. ACOG Practice Bulletin No. 180 (2018); includes LGA risk assessment. — American College of Obstetricians and Gynecologists (2018)

Methodology & trust

Editorial

familia calculator reviewed by Martín Rodríguez, editor of Hacé Cuentas, checked against Hadlock FP, Harrist RB, Carpenter RJ, et al. Sonographic estimation of fetal weight. The value of the femur length in addition to head and abdomen measurements. Radiology. 1984;150(2):535–540., following our editorial policy and methodology.

Updates

Last reviewed: June 2026. Parameters are verified periodically against the cited sources.

Privacy

Calculations run 100% in your browser. We do not store or transmit your data.

Limitations

Indicative results. For critical decisions, consult a professional.

📌 How to cite this calculator

Rodríguez, M. (2026). Fetal Weight by Week (Percentile). Hacé Cuentas. https://hacecuentas.com/en/fetal-weight-percentile-by-week

Content licensed under CC-BY 4.0 — reuse it citing the source with a link to Hacé Cuentas.

✉️ Report an error in this calculator