Fetal Weight Percentile Calculator
Enter the four fetal biometric measurements from your ultrasound report and the gestational age. The calculator uses the Hadlock 4-parameter formula, the clinical standard built into most ultrasound machines, to compute the Estimated Fetal Weight (EFW) and shows where that weight falls on the Hadlock growth percentile curve. You can see the full show-your-work breakdown, reference percentile weights for your gestational week, and a growth chart across all 40 weeks. This is an educational tool: always review results with your obstetrician or midwife.
Formula
Worked example
At 32 weeks with BPD = 8.1 cm, HC = 29.9 cm, AC = 28.5 cm, FL = 6.2 cm: log10(EFW) = 1.3596 - 0.00386*(28.5*6.2) + 0.0064*29.9 + 0.00061*(8.1*28.5) + 0.0424*28.5 + 0.174*6.2 = 1.3596 - 0.6821 + 0.1914 + 0.1408 + 1.2084 + 1.0788 = 3.2969, so EFW = 10^3.2969 = 1981 g. The Hadlock 32-week median is 1953 g, so this sits just above the median at about the 55th percentile.
What is the Estimated Fetal Weight percentile?
The Estimated Fetal Weight (EFW) percentile tells you how a baby's ultrasound-estimated weight compares to a reference population at the same gestational age. A percentile of 50 means the baby weighs about the same as the median. A percentile below 10 means the baby is lighter than 90% of babies at that week of pregnancy, while a percentile above 90 means the baby is heavier than 90% of babies. These thresholds, specifically the 10th and 90th percentiles, are the internationally recognised clinical boundaries for Small for Gestational Age (SGA) and Large for Gestational Age (LGA) designations, as defined by major bodies including ACOG and FIGO.
How is fetal weight estimated and the percentile calculated?
This calculator uses the Hadlock 4-parameter formula, the most widely validated and clinically adopted equation in obstetrics. The formula combines four biometric measurements captured during a standard fetal anatomy ultrasound: Biparietal Diameter (BPD, the width of the skull), Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL). The AC measurement is the most sensitive single predictor of fetal growth restriction because the fetal liver, the organ most affected by malnutrition, contributes heavily to the abdominal measurement. Once the EFW in grams is computed, the percentile is derived by comparing the log10 of the EFW to the Hadlock (1991) reference distribution for the given gestational week, which has a published mean and standard deviation. The z-score tells you how many standard deviations above or below the median the baby sits.
How to enter measurements from your ultrasound report
Your ultrasound report will list the four measurements under the biometry section, often abbreviated as BPD, HC, AC, and FL. Values are usually given in millimetres (mm) on the report: to convert to centimetres, divide by 10 (for example, 81 mm = 8.1 cm). Enter the gestational age as whole weeks. If your report says 31+4 weeks, enter 32 (the nearest complete week). You can switch between metric (cm) and imperial (inches) using the unit selector: the calculator converts internally before applying the formula. The result updates as you type, and the "Show your work" panel traces every step of the calculation with your actual numbers.
What do SGA, AGA, and LGA mean?
- SGA (Small for Gestational Age): EFW below the 10th percentile. This does not automatically mean the baby is unhealthy; some babies are constitutionally small. However, it can indicate Intrauterine Growth Restriction (IUGR), where the baby is not reaching its growth potential, often due to placental insufficiency. Further monitoring with Doppler studies and serial biometry is typically recommended.
- AGA (Appropriate for Gestational Age): EFW between the 10th and 90th percentiles. Growth is within the expected range. A large majority of fetuses fall here.
- LGA (Large for Gestational Age): EFW above the 90th percentile. This may be constitutional (the parents are large) or related to gestational diabetes or other factors. LGA fetuses can carry risks of shoulder dystocia and birth complications, so management options are usually discussed with the care team.
Limitations and accuracy of EFW
Sonographic EFW is inherently imprecise. Published studies consistently report a systematic error of roughly +/-10-15% (about 1 standard deviation) compared to the actual birthweight, even with optimal technique and equipment. A single scan is therefore a snapshot: a trend across multiple scans spaced 2-4 weeks apart is far more informative than any individual reading. Other important caveats include: EFW accuracy declines in late pregnancy and with higher BMI; the Hadlock formula was derived from a predominantly North American population and may not be perfectly calibrated for all ethnicities; amniotic fluid volume and fetal position affect image quality and measurement accuracy; and gestational age dating (best done with a first-trimester crown-rump length measurement) directly affects the percentile calculation. Always discuss results with a qualified perinatal specialist who can interpret findings in the full clinical context.
Hadlock fetal weight reference by gestational age
| Week | 3rd %ile (g) | 10th %ile (g) | 50th %ile (g) | 90th %ile (g) | 97th %ile (g) | Category |
|---|---|---|---|---|---|---|
| 14 | 73 | 79 | 93 | 109 | 118 | AGA: 79-109 |
| 16 | 115 | 124 | 146 | 172 | 185 | AGA: 124-172 |
| 18 | 176 | 189 | 223 | 262 | 283 | AGA: 189-262 |
| 20 | 261 | 281 | 331 | 389 | 420 | AGA: 281-389 |
| 22 | 376 | 406 | 478 | 562 | 606 | AGA: 406-562 |
| 24 | 527 | 569 | 670 | 788 | 850 | AGA: 569-788 |
| 26 | 719 | 776 | 913 | 1075 | 1160 | AGA: 776-1075 |
| 28 | 953 | 1029 | 1210 | 1424 | 1537 | AGA: 1029-1424 |
| 30 | 1228 | 1325 | 1559 | 1835 | 1980 | AGA: 1325-1835 |
| 32 | 1538 | 1660 | 1953 | 2298 | 2480 | AGA: 1660-2298 |
| 34 | 1872 | 2020 | 2377 | 2798 | 3019 | AGA: 2020-2798 |
| 36 | 2216 | 2391 | 2813 | 3311 | 3572 | AGA: 2391-3311 |
| 38 | 2549 | 2750 | 3236 | 3809 | 4110 | AGA: 2750-3809 |
| 40 | 2850 | 3076 | 3619 | 4259 | 4596 | AGA: 3076-4259 |
Estimated fetal weight (grams) at the 3rd, 10th, 50th, 90th, and 97th percentiles. Source: Hadlock 1991 reference curves, widely adopted by ACOG.
Frequently asked questions
Is a fetal weight below the 10th percentile always dangerous?
Not necessarily. Below the 10th percentile is the clinical definition of Small for Gestational Age, but not every SGA fetus has a problem. Some babies are constitutionally small (especially if the parents are small), while others are truly growth-restricted due to placental issues. Your provider will typically look at the trend over multiple scans, Doppler flow studies, and other clinical factors before drawing any conclusions.
Which biometric measurement matters most for detecting growth restriction?
The Abdominal Circumference (AC) is the single most sensitive measurement for detecting fetal growth restriction. The fetal liver shrinks when nutrients are limited because the body diverts resources to the brain and heart, reducing glycogen stores in the liver. This preferential diversion is reflected as a reduced AC relative to head circumference, a pattern sometimes called "head-sparing."
How accurate is the Hadlock 4-parameter formula?
The Hadlock 4-parameter formula is the most widely validated formula in obstetrics, with a typical systematic error of about +/-10-15% compared to actual birthweight. In practical terms, for an estimated weight of 2000 g, the actual birth weight is likely somewhere between 1700 and 2300 g. This is why serial biometry and clinical context matter more than any single number.
My measurements are in millimetres. How do I convert them?
Simply divide by 10. For example, if your report shows BPD = 81 mm, HC = 299 mm, AC = 285 mm, FL = 62 mm, enter 8.1, 29.9, 28.5, and 6.2 in the cm fields. Alternatively, switch to imperial (inches) mode and divide your mm values by 25.4.
What does the z-score mean in this calculator?
The z-score represents how far the estimated fetal weight is from the median for that gestational age, expressed in standard deviations. A z-score of 0 means exactly average, -1.28 corresponds to the 10th percentile, and +1.28 corresponds to the 90th percentile. A z-score below -2 (roughly the 2.3rd percentile) or above +2 (roughly the 97.7th percentile) is generally considered clinically significant and warrants close follow-up.
Can I use this calculator for twins or multiples?
This calculator uses the Hadlock singleton reference curves, which are not validated for twin or higher-order multiple pregnancies. Multiples tend to have lower weights at each gestational age than singletons, and specialist twin growth charts (such as those by Stirrup et al. 2017 or the Twins and Multiple Births Association guidelines) are more appropriate. Enter individual measurements for each twin only as a rough guide, and always review with your specialist.
How does gestational age affect the percentile result?
Gestational age is the reference point for the entire calculation: the same EFW in grams can be the 80th percentile at 30 weeks and the 10th percentile at 36 weeks, because the reference distribution shifts upward as pregnancy progresses. This is why accurate gestational age dating, ideally from a first-trimester crown-rump length measurement, is critical. An error of even one week can shift the percentile result meaningfully.