Fetal Weight by Week (Percentile)
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
How it works
2 min readWhat 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:
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
Hadlock vs. Other Formulas
This calculator uses Hadlock's regression coefficients as the standard of care in prenatal imaging.
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 and 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.
- American College of Obstetricians and Gynecologists (ACOG). Intrauterine Growth Restriction. Practice Bulletin No. 204. Obstet Gynecol. 2019;133(2):e97–e109.
- 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.
- 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.
- Society for Maternal–Fetal Medicine. Screening and diagnosis of gestational diabetes mellitus. ACOG Practice Bulletin No. 180 (2018); includes LGA risk assessment.