Estimate gestational age and fetal weight based on ultrasound (USG) measurements.
Uses the Hadlock formula for CRL (first trimester), BPD and FL (second and third trimesters). Estimated fetal weight (EFW) from 4 ultrasound parameters. Reference table of normal sizes per week.
Disclaimer: This estimate is not a substitute for a doctor's examination. Always discuss ultrasound results with your obstetrician.
Calculator information
📋 How to use this calculator
- Select the available ultrasound parameter: CRL (Crown-Rump Length) for trimester 1, BPD/HC/FL for trimesters 2-3.
- Enter the measurement in millimeters (mm) or centimeters (cm) as reported on the ultrasound report.
- For estimated fetal weight (EFW), enter all four parameters: BPD, HC, AC, FL.
- View the gestational age (GA) and the estimated due date (EDD).
- Compare fetal measurements with the normal reference table for each week of pregnancy.
- Consult your doctor if measurements fall below the 10th percentile or above the 90th percentile.
🧮 Hadlock Estimated Fetal Weight (4 parameters)
log10(EFW) = 1.3596 - 0.00386 * AC * FL + 0.0064 * HC + 0.00061 * BPD * AC + 0.0424 * AC + 0.174 * FL
- EFW: Estimated Fetal Weight (grams)
- BPD: Biparietal Diameter (cm)
- HC: Head Circumference (cm)
- AC: Abdominal Circumference (cm)
- FL: Femur Length (cm)
- CRL T1: GA (days) = 8.052 * sqrt(CRL_mm) + 23.73
The Hadlock 1985 formula for EFW has an average error of +/- 10-15%. In trimester 1, CRL is the most accurate measurement with a deviation of only +/- 3-5 days.
💡 Worked example: Second-trimester ultrasound at 24 weeks gestation
Given:- BPD: 6.0 cm (60 mm)
- HC: 22.0 cm
- AC: 19.5 cm
- FL: 4.4 cm (44 mm)
Steps:- Compute log10(EFW): 1.3596 - 0.00386 * 19.5 * 4.4 + 0.0064 * 22 + 0.00061 * 6 * 19.5 + 0.0424 * 19.5 + 0.174 * 4.4
- = 1.3596 - 0.3312 + 0.1408 + 0.0714 + 0.8268 + 0.7656
- = 2.833
- EFW = 10^2.833 = 681 grams
- GA estimate from BPD (Hadlock): GA ~ 24 weeks
Result: Estimated fetal weight: 681 grams at 24 weeks gestation, consistent with the 50th percentile for that week.
❓ Frequently asked questions
Why is CRL the most accurate measurement in the first trimester?
CRL (Crown-Rump Length) is the length from the crown of the head to the rump of the fetus. First-trimester growth is highly consistent across individuals because it is not yet significantly influenced by nutrition or genetics. CRL provides GA accuracy of +/- 3-5 days at weeks 7-13. After 14 weeks, growth variation emerges so BPD and FL become primary parameters with lower accuracy (+/- 7-10 days).
What is the difference between LMP-based GA and ultrasound GA?
LMP (Last Menstrual Period) gives GA assuming a 28-day cycle and day-14 ovulation. For women with irregular cycles, LMP can be off by 2-3 weeks. Early ultrasound (before 14 weeks) using CRL is considered the 'gold standard' and serves as the GA reference going forward. If the LMP-vs-ultrasound difference is >7 days in T1, use the ultrasound GA.
Is the estimated fetal weight (EFW) always accurate?
EFW has a standard error of +/- 10-15%, meaning a fetus estimated at 3,000 grams could be born anywhere between 2,550-3,450 grams. Accuracy decreases in late third trimester and for very large or very small fetuses. The Hadlock formula is most popular; alternative formulas (Shepard, Warsof) are sometimes used. Actual birth weight is only certain after delivery.
What is the EDD and how is it calculated?
EDD (Estimated Due Date) is the projected date of delivery. Naegele's rule: LMP + 7 days - 3 months + 1 year, or simply LMP + 280 days. Only 5% of babies are born exactly on the EDD; the normal range is 37-42 weeks (full-term). Delivery before 37 weeks is preterm; after 42 weeks is post-term and risky, often requiring induction.
What are the risks if fetal size is below the 10th percentile?
Measurements below the 10th percentile are called SGA (Small for Gestational Age) or IUGR (Intrauterine Growth Restriction) when accompanied by evidence of impaired growth. Causes vary: placental insufficiency, maternal hypertension, infection (TORCH), or chromosomal anomalies. Doctors typically recommend umbilical artery Doppler ultrasound, fetal heart rate monitoring, and possibly early delivery if perfusion worsens.
📚 Sources & references
Last updated: May 11, 2026