EFW Percentile in Pregnancy: Fetal Biometry, AC, and Growth Charts (2026)

EFW percentile in pregnancy: how estimated fetal weight is calculated from biometry measurements, and what low or high percentiles mean clinically.

The EFW percentile in pregnancy is one of the most referenced numbers in prenatal monitoring, and one of the most misread. EFW stands for estimated fetal weight, and the value is not directly measured. It is a calculated estimate derived from ultrasound measurements of specific body parts, run through a formula. The percentile next to it places that estimate within a reference population at the same gestational age.

Two fetuses at the 28th percentile are not in the same situation if one has been tracking there since 20 weeks and the other dropped from the 58th percentile in four weeks. The number alone does not carry that context. The Baby Percentile Calculator applies INTERGROWTH-21st and WHO reference standards to any measurement. This guide covers how EFW is estimated, what each biometry measurement contributes, INTERGROWTH-21st reference values by week, why abdominal circumference gets separate clinical attention, and what low or high percentiles indicate in practice.

What EFW Percentile Is and How Ultrasound Estimates Fetal Weight

EFW is calculated from ultrasound biometry measurements using a regression formula. The most widely used is the Hadlock model, which takes three to four measurements and produces an estimated weight in grams. The most common Hadlock variant uses head circumference (HC), abdominal circumference (AC), and femur length (FL):

log10(EFW) = 1.326 + 0.0107 × HC + 0.0438 × AC + 0.158 × FL - 0.00326 × AC × FL

The output from the equation is a log-transformed value that is then converted to grams. Other formulas (Shepard, INTERGROWTH-21st, Hadlock with BPD) use slightly different inputs and weightings. Two ultrasounds performed on the same day by different operators using different formulas can yield different EFW values without either being incorrect. Most hospital reporting systems default to Hadlock.

The percentile:

Once EFW is calculated, it is plotted against a reference chart for that gestational age. The percentile reflects where the estimated weight sits within the reference population. A 40th percentile EFW at 30 weeks means the baby's estimated weight is higher than 40% of fetuses in the reference population at 30 weeks. Normal range is conventionally the 10th to 90th percentile.

The reference population matters significantly. INTERGROWTH-21st, the most widely cited international standard, was derived from a prospective study of healthy pregnancies across eight countries under favorable conditions: adequate nutrition, no smoking, quality prenatal care. Hadlock's original charts came from a predominantly white US sample in the 1980s. The same EFW can fall at different percentiles on different charts. Clinical units typically specify which standard they use in the scan report.

Accuracy margin:

EFW carries an inherent margin of approximately 15 to 20% in either direction. A reported EFW of 1,500 g could reflect an actual weight anywhere from about 1,200 to 1,800 g. Accuracy is lower in late pregnancy when the fetus is larger and harder to capture fully, and at the extremes of the weight distribution. This margin is not a flaw in the measurement; it reflects the limits of estimating weight from two-dimensional cross-sections of a three-dimensional structure. EFW is more reliable as a trend indicator across serial scans than as a precise single-point number.

The Four Biometry Measurements: BPD, HC, AC, and FL

Each measurement targets a different structure, reflects different aspects of growth, and diverges from the others in different clinical scenarios. Understanding what each one represents explains why the percentile on an individual measurement sometimes matters as much as the composite EFW.

BPD: Biparietal Diameter

BPD measures the width of the fetal skull at its widest cross-section, from one outer surface to the other. It was historically central to gestational age dating before detailed anatomy scans became standard, reliable from about 12 weeks. BPD is now less central to EFW formulas than HC because skull circumference captures head geometry more completely, particularly in fetuses with non-round skull shapes. Dolichocephaly (elongated skull) and brachycephaly (flattened skull) affect BPD meaningfully but not HC. BPD remains a standard scan measurement and is included in many EFW formulas alongside HC.

HC: Head Circumference

HC measures the perimeter of the outer skull. It is more reproducible than BPD because it is less sensitive to skull shape variation, and it reflects both brain volume and overall fetal size. HC diverging from other parameters carries specific implications: a fetus with a small HC and a proportionally normal AC is more likely to have a structural or chromosomal cause for growth abnormality than one with a small AC and a normal HC. The pattern of which measurements are affected, not just the composite EFW, guides the differential.

AC: Abdominal Circumference

AC measures the perimeter of the fetal abdomen at the level of the portal venous system. This plane captures the liver, which is the largest organ in the fetal body and a major site of glycogen storage. AC is the most sensitive biometry measurement for detecting nutritional growth restriction, for one specific reason: when placental nutrient delivery falls short, the fetus prioritizes brain and cardiac perfusion over visceral growth. Hepatic glycogen stores deplete, the liver shrinks relative to other organs, and AC falls before HC or FL register significant change.

AC is also the most sensitive parameter for macrosomia. In pregnancies complicated by gestational diabetes, excess glucose drives fetal hyperinsulinism and hepatic fat deposition, enlarging the abdomen disproportionately. AC crossing above the 90th percentile while other measurements remain lower is the typical early pattern of diabetic macrosomia.

FL: Femur Length

FL measures the length of the femoral bone shaft. It reflects skeletal growth and correlates with overall body length. FL is less sensitive to short-term nutritional changes than AC, which makes it useful as a stable reference: when AC drops sharply but FL remains on its prior trajectory, the asymmetry points to an acquired nutritional problem rather than a constitutional or structural one. Isolated short FL within an otherwise proportionate fetus is commonly a normal variant, particularly in populations with shorter stature. Significant FL shortening merits evaluation for skeletal dysplasias, but mild shortening in isolation is rarely actionable.

For a broader overview of how these parameters map to clinical decisions at each trimester scan, the Baby Percentile During Pregnancy guide covers the trimester-by-trimester interpretation framework and what changes in percentile across scans indicate.

EFW Percentile by Gestational Week: INTERGROWTH-21st Reference Values

The table below shows approximate EFW values at the 10th, 50th, and 90th percentile lines from the INTERGROWTH-21st Fetal Growth Standards. These values represent the most widely used international reference for fetal weight from 20 to 40 weeks.

Values are approximate from INTERGROWTH-21st Fetal Growth Standards. Individual units may apply Hadlock or NICHD charts, which produce slightly different cut-offs.

The range between the 10th and 90th percentile is wider than most parents expect. At 28 weeks, a fetus at the 10th percentile weighs about 815 g while one at the 90th percentile weighs about 1,376 g. Both are within the normal reference range. The percentile is not a performance score.

Growth velocity across scans:

A single percentile from one scan is less informative than the trajectory across two or three scans. A fetus consistently tracking at the 15th percentile across all scans has a different clinical picture from one that measured at the 50th percentile at 24 weeks, 35th at 28 weeks, and 15th at 32 weeks, even though both are at the 15th percentile at the most recent scan. The drop in percentile points over time, known as growth deceleration, is what triggers closer monitoring rather than the absolute percentile position alone.

Gestational age accuracy also affects where a baby falls on the chart. An EFW that appears low partly because the pregnancy dates are off by 10 days looks significantly different once dates are corrected. When gestational age is uncertain, early first-trimester dating by crown-rump length is the most accurate reference, and later scan measurements are interpreted against that date.

For reference values used after birth, the Baby Weight Percentile Chart by Age covers WHO growth standards by sex and age in months, which differ from the prenatal reference charts.

AC Percentile in Pregnancy: Why Abdominal Circumference Gets Separate Attention

Most scan reports show individual percentiles for each biometry measurement alongside the composite EFW percentile. Among the individual measurements, AC carries specific clinical significance that the other three do not carry in the same way.

Why AC diverges first in growth restriction:

When uteroplacental blood flow is reduced, the fetal brain receives preferential perfusion at the expense of the viscera. This is called brain-sparing, and it is a compensatory response to chronic hypoxia and nutrient deprivation. The liver, the largest organ and the one most dependent on glucose delivery for glycogen synthesis, shows the effects earliest. AC falls before HC or FL because the liver is shrinking while the brain is being protected.

This pattern is called asymmetric intrauterine growth restriction (IUGR). An AC at the 8th percentile while HC remains at the 40th percentile in a fetus with abnormal umbilical artery Doppler is a recognizable presentation of late-onset placental insufficiency. A fetus with all four parameters symmetrically at the 8th percentile more often reflects constitutional smallness, early-onset restriction, or a chromosomal cause.

Clinical response to low AC percentile:

When AC falls below the 10th percentile in isolation with a normal EFW, most protocols increase scan frequency to monitor growth velocity. When low AC combines with other abnormal features (reduced liquor, abnormal Doppler waveforms, maternal risk factors for growth restriction), management escalates further.

An AC below the 3rd percentile, particularly with evidence of Doppler deterioration, triggers a much more active surveillance and delivery planning protocol than an isolated AC at the 8th percentile with normal Doppler and normal liquor.

High AC percentile:

AC above the 90th percentile in the third trimester is the most sensitive biometry indicator of gestational diabetes. The abdominal enlargement often precedes measurable changes in overall EFW percentile. In pregnancies with no known diabetes, a disproportionately elevated AC relative to HC and FL prompts glucose screening if not already completed.

The practical implication: when a scan report shows a composite EFW in a normal range but an AC percentile significantly higher or lower than the other measurements, the asymmetry is the finding. It should prompt a review of the Doppler, liquor, and maternal history rather than reassurance based on the EFW alone.

What Low and High EFW Percentiles Mean Clinically

EFW percentile is a screening parameter, not a diagnosis. Its value is in identifying which pregnancies need more assessment, not in determining outcomes on its own.

Below the 10th percentile: SGA and FGR

Small for gestational age (SGA) is defined as EFW or birth weight below the 10th percentile for gestational age. SGA is a statistical definition. Many fetuses below the 10th percentile are constitutionally small, growing consistently along a low percentile line with normal placental function, normal Doppler, and no evidence of nutrient deprivation. They are small in the same way some adults are short: within the range of normal biological variation.

Fetal growth restriction (FGR) implies a fetus is failing to reach its genetic growth potential due to an underlying pathological cause, most commonly placental insufficiency. FGR is suspected when SGA is accompanied by reduced growth velocity across serial scans, abnormal umbilical artery or middle cerebral artery Doppler, reduced amniotic fluid, or clinical risk factors for placental disease.

SGA without any FGR features is generally managed with increased scan surveillance but not early delivery in the absence of deteriorating parameters. FGR with abnormal Doppler, particularly with absent or reversed end-diastolic flow in the umbilical artery, triggers intensive monitoring and may result in planned delivery before term when the intrauterine risk exceeds the risk of prematurity.

Below the 3rd percentile:

The 3rd percentile carries different management thresholds from the 10th in most fetal medicine guidelines. Regardless of Doppler findings, an EFW below the 3rd percentile is associated with a higher baseline risk of adverse perinatal outcome and is typically managed with the same intensity as an EFW between the 3rd and 10th percentile with an additional abnormal finding.

Above the 90th percentile: LGA and macrosomia

Large for gestational age (LGA) is EFW above the 90th percentile. Clinical macrosomia is typically defined as EFW above 4,000 to 4,500 g at term depending on the institution. LGA is associated with shoulder dystocia, operative delivery, and birth trauma during vaginal birth, with risk increasing substantially above 4,500 g.

The clinical response to LGA depends on context. An isolated 92nd percentile EFW with no diabetes history, normal amniotic fluid, and proportionate biometry is monitored differently from a 96th percentile EFW with a disproportionately enlarged AC in a pregnancy with gestational diabetes. The etiology, not just the number, shapes the management decision.

Serial tracking above everything:

A fetus that drops from the 60th percentile to the 25th percentile over eight weeks warrants more attention than a fetus consistently at the 12th percentile across all scans, even though the latter has the lower absolute percentile. Growth deceleration across a reference population can signal deteriorating placental function before the EFW crosses below any diagnostic threshold.

For the statistical framework behind percentile interpretation and how the same percentile number is read differently before and after birth, the What Does Baby Percentile Mean guide covers the reference standards, normal variation, and how clinicians use growth charts across the full age range.