Fetal Weight Percentile Calculator

Understanding Percentiles in Fetal Growth

Percentiles measure where a specific measurement falls within a population distribution. If your baby ranks at the 60th percentile, that means 60% of babies at the same gestational age weigh less, and 40% weigh more. This statistical approach accounts for the natural variation in fetal development without imposing arbitrary absolute values.

Clinicians rely on percentile rankings because they provide immediate context. Rather than saying a baby weighs 1,200 grams (which could be normal or concerning depending on when in pregnancy it occurs), percentiles instantly communicate whether growth is tracking appropriately. The 10th to 90th percentile range is generally considered normal, though individual growth trajectories matter as much as single measurements.

Serial measurements across multiple scans reveal growth velocity—whether a baby is maintaining a consistent percentile or crossing percentile lines. A baby who moves from the 50th to the 10th percentile between two scans requires clinical attention, even if both weights fall within the technically normal range.

The Hadlock Fetal Weight Equation

The Hadlock formula combines four ultrasound parameters in a logarithmic equation to estimate fetal weight. This multivariate approach provides superior accuracy compared to single-parameter methods, particularly across the second and third trimesters. Take your measured values, apply the formula, and convert the result back from logarithmic form to get weight in grams.

• AC — Abdominal circumference in millimetres, measured at the level of the liver
• HC — Head circumference in millimetres, measured around the widest part of the fetal skull
• BPD — Biparietal diameter in millimetres, the distance between the two parietal bones
• FL — Femur length in millimetres, measured from the hip joint to the knee
• Weight — Estimated fetal weight in grams, derived by taking 10 to the power of the calculated log value

How to Enter Your Ultrasound Measurements

Start by recording all four parameters from your ultrasound report. Ensure they are in millimetres—if your report uses centimetres, multiply by 10 before entering. Specify your baby's sex if known, as male and female fetuses follow slightly different growth curves. Enter the gestational age in completed weeks; this calculator applies to pregnancies at 14 weeks or older.

The calculator will compute estimated weight and automatically cross-reference it against WHO multinational growth standards and Hadlock reference data to generate your percentile rank. Pay attention to the feedback provided: measurements near the 10th or 90th percentile warrant discussion with your clinician, as may rapid changes from one scan to the next.

Always ensure your sonographer confirms which measurements correspond to which parameters. Mislabeled values will produce incorrect weight estimates. If you receive measurements at multiple scans, track the percentile trend rather than fixating on a single data point.

Practical Considerations for Percentile Interpretation

Percentile rankings provide context but require careful interpretation alongside clinical judgment.

1. Individual variation is normal — Healthy babies span the full percentile range. A baby at the 5th percentile is not automatically at risk, nor is a baby at the 95th percentile automatically large. Growth consistency and appropriate velocity matter more than absolute percentile placement. Discuss any concerns with your obstetric team rather than self-diagnosing based on percentile rank alone.
2. Measurement technique affects accuracy — Ultrasound image quality, operator experience, and exact plane of measurement all influence whether AC, HC, BPD, and FL are accurately captured. Rescanning or obtaining images at a tertiary centre may refine estimates if there is clinical uncertainty. Small measurement errors can produce noticeable percentile shifts, so serial scans from the same facility often provide better trend data.
3. Formula reliability varies by gestational age — The Hadlock equation performs best between 14 and 42 weeks. Before 14 weeks, crown-rump length is the preferred parameter. Growth estimates have inherent variability—typically ±15% accuracy—so a calculated weight of 1,000 grams could realistically range from 850 to 1,150 grams at delivery.
4. Percentile charts assume uncomplicated pregnancies — Maternal diabetes, hypertension, previous small-for-date infants, or other risk factors may shift expected growth patterns. If you have known complications, discuss what your personalized percentile targets should be with your clinician rather than relying on population averages.

Reference Ranges by Gestational Age

Fetal weight varies substantially across pregnancy. At 14 weeks, median weight is approximately 90 grams; by 20 weeks it reaches 330 grams; and by 28 weeks it approaches 1,000 grams. By 36 weeks, most babies weigh between 2,200 and 2,900 grams depending on percentile rank. At term (40 weeks), median weight is around 3,400 to 3,500 grams, with the normal range spanning from roughly 2,500 grams (10th percentile) to 4,500 grams (90th percentile).

These reference values emerge from large databases of measured newborn weights and are integrated into growth charts used worldwide. Your baby's percentile should reflect both estimated weight and gestational age simultaneously. A weight that appears normal at 36 weeks would be concerning at 32 weeks—context is essential.