👶 Fetal Weight Percentile Calculator

Estimated fetal weight comes from the Hadlock formula using four ultrasound measurements; the percentile compares it to typical weights for the week. At 36 weeks, BPD 8.8, HC 32, AC 30, FL 6.8 cm gives EFW ~2,463 g, about the 29th percentile (median ~2,614 g). Enter your biometry for your estimate.

How to Use This Calculator

The four measurements come from the ultrasound report, in centimeters. The gestational age should be the ultrasound-dated age, since percentile depends entirely on the week.

The Hadlock Formula

Hadlock and colleagues published this four-parameter formula in 1985, and it remains the most used EFW equation because it balances accuracy with availability: all four measurements are routine on growth scans. The AC term dominates because the abdomen reflects liver size and fat stores, the tissues that vary most with growth. Published accuracy is roughly ±10-15% (about ±1 standard deviation), meaning a 3,000 g estimate could truly be 2,600-3,400 g. That error band is the most important thing on this page: EFW is an estimate with real uncertainty, and clinical decisions use trends and additional testing, not single numbers.

Worked Example

36 weeks: BPD 8.8, HC 32.0, AC 30.0, FL 6.8. Plugging into the formula: log10(EFW) = 1.326 − 0.665 + 0.342 + 1.314 + 1.074 = 3.392, so EFW = 10^3.392 ≈ 2,463 g (5.43 lb). Median at 36 weeks ≈ 2,614 g. z = (ln 2463 − ln 2614)/0.11 ≈ −0.54, about the 29th percentile: appropriate for gestational age, mildly below median. I verified the calculator reproduces this exact computation. The percentile uses a lognormal model with 11% coefficient of variation, approximating standard growth charts.

Percentile Bands

Percentile is not destiny: a fetus tracking steadily along the 12th percentile is usually fine, while one falling from the 60th to the 15th across scans suggests growth restriction even though 15th is "normal." Trend beats snapshot, which is why growth concerns get serial scans 2-3 weeks apart. Doppler studies of the umbilical artery separate constitutionally small babies from placentally restricted ones. For the pregnancy timeline around these scans, our due date calculator dates the pregnancy, our fundal height tracker follows the bedside measurement between scans, and our gestational age calculator converts any dating method to weeks and days.

What Moves EFW Off Course

Maternal diabetes pushes EFW up (asymmetric: big abdomen, normal head); smoking, hypertension, and placental insufficiency push it down. Dating errors move the percentile more than the weight: a pregnancy dated a week too young makes every weight look large. Oligohydramnios degrades measurement quality. And the ±15% error means borderline calls (9th vs 11th percentile) should never drive solo decisions. When providers act on growth, they act on patterns across scans plus fluid, Dopplers, and maternal context.

After an Abnormal Result

Under 10th percentile typically triggers: confirm dating, umbilical artery Doppler, amniotic fluid check, and repeat growth scan in 2-3 weeks, plus antenatal testing (NST/BPP). Over 90th: review glucose control, consider the delivery plan (shoulder dystocia risk rises with true macrosomia over 4,500 g, lower threshold with diabetes). Neither band means something is wrong with your baby; both mean closer watching, which is how modern obstetrics converts risk into safety. Bring your questions to the maternal-fetal medicine visit; they answer these daily.

Customized vs Population Growth Charts

Standard charts compare every fetus to the same population median, but customized charts adjust for maternal height, weight, ethnicity, and parity, on the theory that a small mother grows a constitutionally smaller baby. Customized charts reclassify some "SGA" fetuses as normal and flag some "normal" ones as restricted. The debate is unresolved: customization reduces false alarms but may normalize truly restricted growth in small mothers. Most US practice still uses population standards (like the Hadlock-based references behind this calculator), while the UK's growth-assessment protocol uses customized charts. Ask which chart your practice uses if a percentile surprises you; the same measurements can read differently.

Always confirm which growth standard your practice uses before comparing a home calculation to the clinic's percentile. Different charts (Hadlock, INTERGROWTH-21st, WHO, customized) can place the same measurements several percentile points apart.

For informational purposes only, not medical advice. Consult a healthcare professional.