Health & Medicine · Obstetrics & Pediatrics
Fetal Weight Estimator
Estimates fetal weight using standard ultrasound biometric measurements via the Hadlock formula.
Calculator
Formula
EFW = estimated fetal weight in grams. HC = head circumference in cm. AC = abdominal circumference in cm. FL = femur length in cm. BPD = biparietal diameter in cm. The result of the log expression is the base-10 logarithm of EFW, so EFW = 10^(result). This is the Hadlock four-parameter formula (1985), the most widely validated model in obstetric practice.
Source: Hadlock FP, et al. Estimation of fetal weight with the use of head, body, and femur measurements — a prospective study. American Journal of Obstetrics and Gynecology. 1985;151(3):333–337.
How it works
Fetal weight cannot be measured directly in utero. Instead, clinicians use diagnostic ultrasound to capture standardized biometric measurements, which are then combined in regression equations to predict weight. The most widely adopted of these is the Hadlock four-parameter formula, published in 1985 and validated across diverse populations. It uses BPD, HC, AC, and FL — measurements that collectively capture the fetal head size, brain volume, abdominal girth (a strong proxy for liver size and soft tissue fat), and skeletal growth.
The Hadlock formula expresses estimated fetal weight as a logarithmic function: log₁₀(EFW) = 1.3596 + 0.0064×HC + 0.0424×AC + 0.174×FL + 0.00061×BPD×AC − 0.00386×AC×FL. All biometric inputs are entered in centimeters, and EFW is returned in grams by computing 10 to the power of the result. The interaction terms (BPD×AC and AC×FL) allow the formula to account for the non-linear relationship between body proportions and actual mass. The formula carries an inherent error margin of approximately ±15–20% in clinical conditions.
Clinically, EFW is plotted against gestational age percentile charts to classify fetal growth status: small for gestational age (SGA) below the 10th percentile, appropriate for gestational age (AGA) between the 10th and 90th percentiles, and large for gestational age (LGA) above the 90th percentile. These classifications inform decisions about antenatal monitoring frequency, timing of delivery, and mode of delivery. EFW tracking across serial scans — typically every 2–4 weeks — is more informative than any single measurement.
Worked example
A sonographer performs a third-trimester ultrasound at 36 weeks gestation and records the following measurements:
- BPD = 9.1 cm
- HC = 32.4 cm
- AC = 33.8 cm
- FL = 7.0 cm
Step 1 — Apply the Hadlock formula:
log₁₀(EFW) = 1.3596 + (0.0064 × 32.4) + (0.0424 × 33.8) + (0.174 × 7.0) + (0.00061 × 9.1 × 33.8) − (0.00386 × 33.8 × 7.0)
= 1.3596 + 0.2074 + 1.4331 + 1.218 + 0.18753 − 0.91247
= 3.494
Step 2 — Convert from log to grams:
EFW = 10^3.494 = approximately 3,117 grams (about 3.12 kg or 6.87 lbs)
Step 3 — Interpret: At 36 weeks, an EFW of ~3,117 g falls near the 75th percentile, indicating appropriate-for-gestational-age growth with no immediate concern for macrosomia or growth restriction. The clinician would continue routine monitoring and schedule a follow-up scan in 2–4 weeks if indicated.
Limitations & notes
The Hadlock formula carries a systematic error of ±7.5–15% under ideal conditions and ±15–20% in routine clinical settings, meaning a calculated EFW of 3,000 g could reflect actual fetal weight anywhere between roughly 2,400 and 3,600 g. Accuracy decreases at the extremes of gestational age, in fetuses with anatomical anomalies, in pregnancies complicated by oligohydramnios (which reduces ultrasound window quality), and in cases of fetal malpresentation that make standard measurement planes difficult to obtain. Body mass index of the mother can also impair image quality and measurement accuracy. This calculator is intended as a clinical decision-support tool only and must not be used as the sole basis for obstetric management decisions. All measurements should be obtained by a qualified sonographer using calibrated equipment and standardized scanning protocols. Gestational age should be established accurately — ideally by first-trimester crown-rump length — before interpreting EFW percentiles. No formula-based estimator replaces skilled clinical judgment and comprehensive fetal assessment.
Frequently asked questions
What is the most accurate formula for estimating fetal weight?
The Hadlock four-parameter formula (BPD, HC, AC, FL) is the most widely validated and clinically adopted model for estimated fetal weight. It consistently outperforms simpler two- or three-parameter formulas across a broad range of gestational ages and fetal sizes. Other formulas such as Shepard (AC + BPD) or Warsof are used in specific contexts but are generally considered less accurate.
How accurate is ultrasound fetal weight estimation?
Ultrasound EFW has an inherent error of approximately ±15–20% in clinical practice. This means a fetus estimated at 3,000 g could actually weigh between about 2,400 and 3,600 g. Accuracy is highest between 24–36 weeks gestation and diminishes near term and in technically challenging scans. Serial measurements are more reliable for tracking growth trends than any single estimate.
What is a normal fetal weight by gestational week?
Fetal weight varies considerably by gestational age. General reference values include approximately 500 g at 22 weeks, 1,000 g at 27 weeks, 1,500 g at 31 weeks, 2,000 g at 34 weeks, 2,500 g at 36 weeks, and 3,300–3,500 g at 40 weeks. A fetus is considered growth-restricted below the 10th percentile and large for gestational age above the 90th percentile for their gestational age.
What does abdominal circumference measure in fetal weight estimation?
Abdominal circumference (AC) is the single most important biometric parameter for estimating fetal weight and detecting growth abnormalities. It reflects the size of the fetal liver and the amount of subcutaneous adipose tissue, both of which are highly sensitive to nutritional status and placental function. A falling AC percentile is often the first ultrasound sign of intrauterine growth restriction.
When should fetal growth scans be performed?
Routine growth scans are typically performed at 18–22 weeks (anatomy scan) and again at 28–36 weeks if there are risk factors for growth restriction or macrosomia. High-risk pregnancies — including those with hypertension, diabetes, multiple gestation, or prior IUGR — may require scans every 2–4 weeks from the second trimester onward. Low-risk pregnancies may not require additional growth scans beyond the standard anatomy scan.
Last updated: 2025-01-15 · Formula verified against primary sources.