What is a fetal weight percentile?

A fetal weight percentile tells you how an estimated fetal weight (EFW) from an ultrasound compares with a reference population of fetuses at the same gestational age. The 50th percentile is the median; a 33rd percentile means the estimated weight is heavier than about 33 out of 100 fetuses at that age and lighter than about 67.

How is estimated fetal weight (EFW) calculated?

This calculator uses the Hadlock 1985 four-parameter formula, the most widely used EFW equation in obstetric ultrasound: log10(EFW) = 1.3596 − 0.00386×AC×FL + 0.0064×HC + 0.00061×BPD×AC + 0.0424×AC + 0.174×FL, with BPD, HC, AC and FL in centimetres and the result in grams. It combines head, abdomen and femur measurements into a single weight estimate.

What do SGA, AGA and LGA mean?

SGA (small for gestational age) means the estimated weight is below the 10th percentile; AGA (appropriate for gestational age) means it falls between the 10th and 90th percentiles; LGA (large for gestational age) means it is above the 90th percentile. These are screening classifications, not diagnoses — only your doctor or midwife can interpret what they mean for your pregnancy.

What fetal weight percentile is considered normal?

By convention, estimated weights between the 10th and 90th percentiles (AGA) are considered within the expected range on a population growth chart. A single percentile value is a screening snapshot, not a verdict on your baby's health — growth trends across scans matter more than any one number.

My baby's estimated weight is below the 10th percentile. What does that mean?

An EFW below the 10th percentile is labelled SGA (small for gestational age) as a screening flag. It does not diagnose a problem: many constitutionally small, healthy babies plot below the 10th percentile. Clinicians usually follow up with growth-velocity assessment and sometimes Doppler studies. Discuss the result with your doctor or midwife rather than drawing conclusions from the number alone.

My baby's estimated weight is above the 90th percentile. What does that mean?

An EFW above the 90th percentile is labelled LGA (large for gestational age) as a screening flag. It can be completely normal — especially with a family history of bigger babies — but clinicians may check for gestational diabetes and discuss delivery planning. An EFW above 4000 g is sometimes called macrosomia. Always discuss the result with your doctor or midwife.

How accurate is the Hadlock formula?

Ultrasound weight estimation is approximate: most studies find the majority of Hadlock estimates fall within ±10-15% of actual birth weight, and errors up to ±20% are accepted in clinical practice. Accuracy is worse at the extremes (very small or very large babies) and depends on measurement quality, fetal position and amniotic fluid. Treat every EFW as an estimate with a margin of error, not an exact weight.

Which growth standard does this calculator use?

Percentiles are plotted against the Hadlock 1991 in-utero fetal weight standard (Hadlock, Harrist & Martinez-Poyer, Radiology 1991), built from ultrasound-estimated weights of normally growing fetuses. It gives the median (50th percentile) weight for each gestational week from 16 to 42 weeks with a uniform standard deviation, from which the 3rd, 10th, 90th and 97th centiles are derived.

What is the difference between this and a birthweight percentile calculator?

This tool estimates weight before birth from ultrasound biometry and compares it with an in-utero (fetal) growth standard. A birthweight percentile calculator instead takes the baby's actual weight after delivery and compares it with a newborn standard such as INTERGROWTH-21st. Use our birthweight percentile calculator once the baby is born.

Can I enter measurements in millimetres or inches?

Yes. Use the unit switcher above the inputs to enter biometry in centimetres, millimetres or inches — everything is converted to centimetres internally before the Hadlock formula runs, because the formula requires centimetres. Most ultrasound reports print measurements in millimetres, so divide by 10 or just select mm.

Does this work for twins or triplets?

No. The Hadlock 1991 reference standard was built from singleton pregnancies, so percentiles are only meaningful for one baby at a time. Twins and triplets follow different growth patterns and need twin-specific charts interpreted by a clinician.

At how many weeks of pregnancy can I use this calculator?

The reference table covers 16 weeks 0 days to 42 weeks 6 days of gestation. Weight estimation is most commonly used from the mid-second trimester onward; before about 16 weeks, dating and anatomy assessment matter more than weight percentiles.

What is the difference between this and a due date calculator?

A due date calculator works from dates — your last menstrual period, conception date or IVF transfer — to estimate when the baby will arrive. This tool works from ultrasound body measurements to estimate how much the baby weighs right now. Dates tell you the calendar; biometry tells you the size. Use our pregnancy due date calculator for dating questions.

Should I worry about a single low or high percentile?

A single percentile is a snapshot with measurement error on both sides — the ultrasound estimate itself can be off by 15-20%, and one scan cannot show growth velocity. Clinicians look at the trend across scans, amniotic fluid, Doppler flow and your history before drawing conclusions. If a result worries you, bring the numbers to your doctor or midwife and ask what follow-up, if any, they recommend.

nrtechstudio.com

Fetal Weight Percentile Calculator

Enter ultrasound biometry (BPD, HC, AC, FL) and gestational age to estimate fetal weight with the Hadlock 1985 formula, see the weight percentile against the Hadlock 1991 growth standard, and view the growth curve.

Educational estimate — not medical advice. Always discuss ultrasound results with your doctor or midwife. This tool estimates weight from measurements; it cannot diagnose growth problems.

Ultrasound biometry

Measurement units:

BPD — biparietal diameter Width of the baby's head

HC — head circumference Around the baby's head

AC — abdominal circumference Around the baby's tummy

FL — femur length Thigh bone length

Gestational age

Weeks 16 – 42

Days 0 – 6

🔒 Private by design: every calculation runs inside your browser. Your measurements are never sent anywhere — there is no server, no tracking of inputs.

Result

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Screening classification, not a diagnosis. Percentiles describe where an estimate sits in a reference population — they do not judge your baby's health.

0th25th50th75th100th

Weight percentile

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Expected range (10th–90th) at this age

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Median (50th) at this age

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Gestational age used

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Growth curve — Hadlock 1991 standard

Shaded bands: 3rd–97th (light) and 10th–90th (darker) centiles; the line is the 50th percentile. The dot is your baby's estimated weight at the entered gestational age.

Reference table — Hadlock 1991 (grams)

Median (50th percentile) estimated fetal weight for each gestational week, with the 3rd, 10th, 90th and 97th centiles. Source: Hadlock, Harrist & Martinez-Poyer, Radiology 1991.

Week	3rd	10th	50th	90th	97th

Worked example — 32 weeks

Ultrasound report: BPD 8.0 cm, HC 29.0 cm, AC 28.0 cm, FL 6.0 cm, at 32 weeks 0 days.

Hadlock 1985: log₁₀(EFW) = 1.3596 − 0.00386×28.0×6.0 + 0.0064×29.0 + 0.00061×8.0×28.0 + 0.0424×28.0 + 0.174×6.0 = 3.2646
EFW = 10^3.2646 = 1839 g (≈ 4 lb 0.9 oz)
Hadlock 1991 at 32 weeks: median 1953 g, 10th 1621 g, 90th 2285 g
z = (1839 − 1953) / 259.1 = −0.44 → 33rd percentile → AGA (screening classification, not a diagnosis)

Press “Load worked example” above to run these exact numbers in the calculator.

Methodology & limitations

EFW formula: log10(EFW) = 1.3596 − 0.00386·AC·FL + 0.0064·HC + 0.00061·BPD·AC + 0.0424·AC + 0.174·FL (Hadlock et al., 1985; inputs in cm, output in grams).
Growth standard: Hadlock 1991 in-utero weight standard — median EFW per gestational week (16–42 w) with uniform standard deviation. Centile bounds: p10/p90 = mean ± 1.2816·SD; p3/p97 = mean ± 1.8808·SD. The SD was derived from each week's published (mean, p10) pair and verified uniform across all 27 weeks.
Fractional weeks: for ages like 32w4d, the reference mean and SD are linearly interpolated between the two neighbouring weeks.
Accuracy: ultrasound EFW is approximate — typically ±10–15%, with up to ±20% accepted in clinical practice. Percentiles inherit this uncertainty.
Scope: singleton pregnancies only; 16w0d–42w6d. Percentiles are a screening snapshot — growth velocity across serial scans matters more than a single value.
Privacy: 100% client-side. No measurement ever leaves your browser.

Scope boundary. This tool estimates fetal weight before birth from ultrasound body measurements. For when the baby is due (dates, not size), use our pregnancy due date calculator. For weight percentile after delivery (actual birth weight vs the INTERGROWTH-21st newborn standard), use our birthweight percentile calculator. For fertility timing, see our ovulation calculator.

Frequently asked questions

Free tool by NRTechStudio

Fetal Weight Percentile Calculator

Ultrasound biometry

Gestational age

Result

Growth curve — Hadlock 1991 standard

Reference table — Hadlock 1991 (grams)

Worked example — 32 weeks

Methodology & limitations

Frequently asked questions

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