Fetal Weight Calculator - Estimate Baby Weight by Week

What is it?

Fetal weight estimation is one of the most important assessments during pregnancy. It provides crucial information about your baby's growth and development, helping healthcare providers monitor whether your baby is growing at a healthy rate. The estimated fetal weight (EFW) can be determined through several methods. The most common clinical approach uses ultrasound measurements combined with mathematical formulas, such as the Hadlock formula. Population-based growth charts, like those published by the World Health Organization (WHO), provide reference ranges based on gestational age. Understanding your baby's weight during pregnancy helps identify potential complications early. Babies that are significantly smaller or larger than expected for their gestational age may require additional monitoring or interventions. Small for gestational age (SGA) babies—those below the 10th percentile—may face risks including intrauterine growth restriction (IUGR), while large for gestational age (LGA) babies—those above the 90th percentile—may present delivery challenges. Our fetal weight calculator uses WHO-validated growth charts combined with the Hadlock formula to provide accurate weight estimates at any point during pregnancy, from 8 weeks through full term at 40 weeks.

Formula Details

**The Hadlock Formula (1985)** The Hadlock formula is the most widely used equation for estimating fetal weight from ultrasound measurements. Developed by Dr. Frank P. Hadlock and colleagues at Baylor College of Medicine, it has been validated across diverse populations worldwide. **Full Four-Parameter Equation:** log₁₀(EFW) = 1.326 - 0.00326 × AC × FL + 0.0107 × HC + 0.0438 × AC + 0.158 × FL Where: - EFW = Estimated Fetal Weight in grams - AC = Abdominal Circumference in cm - FL = Femur Length in cm - HC = Head Circumference in cm **Simplified Two-Parameter Equation (AC + FL only):** log₁₀(EFW) = 1.304 + 0.05281 × AC + 0.1938 × FL - 0.004 × AC × FL **WHO Growth Standards:** The World Health Organization published international fetal growth charts in 2017, based on the INTERGROWTH-21st Project, which studied healthy pregnancies across eight countries. These charts provide: - 3rd, 10th, 50th, 90th, and 97th percentile values - Week-by-week growth expectations - Population-independent reference standards **Factors Affecting Fetal Weight:** 1. Maternal nutrition and weight gain 2. Placental function and blood flow 3. Genetic factors (parental size) 4. Gestational age accuracy 5. Multiple pregnancies (twins/triplets tend to be smaller) 6. Maternal health conditions (diabetes, hypertension) 7. Altitude and environmental factors

How to Calculate

There are three main methods to estimate fetal weight: **Method 1: Gestational Week** The simplest approach uses your current gestational week to look up the expected weight from standardized growth charts. This provides the population average (50th percentile) along with normal range boundaries (10th and 90th percentiles). **Method 2: Last Menstrual Period (LMP)** By entering the first day of your last menstrual period, the calculator automatically determines your current gestational week and estimates the corresponding fetal weight. **Method 3: Ultrasound Measurements (Hadlock Formula)** The most accurate clinical method uses four ultrasound biometric measurements: - **BPD (Biparietal Diameter)**: The distance between the two sides of the baby's head - **HC (Head Circumference)**: The circumference around the baby's head - **AC (Abdominal Circumference)**: The circumference around the baby's abdomen - **FL (Femur Length)**: The length of the baby's thigh bone The Hadlock formula (1985) combines these measurements: log₁₀(EFW) = 1.326 - 0.00326×AC×FL + 0.0107×HC + 0.0438×AC + 0.158×FL This formula has been validated in numerous studies and remains the gold standard for ultrasound-based fetal weight estimation, with an accuracy of ±15% in most cases.

Interpretation

**Understanding Your Results:** Your baby's estimated weight is compared against WHO growth percentiles: - **Below 10th percentile (Small for Gestational Age - SGA)**: Your baby may be smaller than expected. While many SGA babies are healthy, this finding warrants close monitoring. Your healthcare provider may recommend additional ultrasounds, non-stress tests, or Doppler studies to ensure adequate blood flow. - **10th to 90th percentile (Appropriate for Gestational Age - AGA)**: Your baby is within the normal weight range. This is where most healthy babies fall, and it indicates appropriate growth and development. - **Above 90th percentile (Large for Gestational Age - LGA)**: Your baby may be larger than average. LGA can be associated with gestational diabetes, genetic factors, or simply family tendency toward larger babies. Additional monitoring may be recommended to plan for safe delivery. **Important Notes on Accuracy:** - Ultrasound weight estimates have a margin of error of ±10-15% - Accuracy decreases in the third trimester as the baby gets larger - A single measurement is less reliable than serial measurements showing a growth trend - Factors like amniotic fluid volume and baby's position can affect measurements

Limitations

**Limitations of Fetal Weight Estimation:** 1. **Inherent Margin of Error**: Even with ultrasound, fetal weight estimates have a ±10-15% margin of error. For a 3,000g baby, this means the actual weight could range from 2,550g to 3,450g. 2. **Operator Dependency**: Ultrasound measurements depend on the sonographer's skill and technique. Different operators may obtain slightly different measurements. 3. **Baby's Position**: If the baby is in an awkward position, certain measurements may be difficult to obtain accurately. 4. **Third Trimester Accuracy**: Estimation accuracy tends to decrease in the late third trimester (after 36 weeks) as the baby's head engages in the pelvis and soft tissue makes measurements less precise. 5. **Maternal Body Habitus**: In mothers with higher BMI, ultrasound image quality may be reduced, potentially affecting measurement accuracy. 6. **Growth Chart Variability**: Different growth charts (WHO, Hadlock, national standards) may give different percentile results for the same weight. 7. **Population Differences**: Growth patterns can vary between ethnic groups and geographic regions, though WHO charts aim to be universal. 8. **Not Diagnostic**: Weight estimation is a screening tool, not a diagnostic test. Clinical decisions should be made in conjunction with other assessments. This calculator is for informational purposes only. Always discuss your baby's growth and development with your healthcare provider, who can interpret results in the context of your complete medical history.

Tips

• Track your baby's weight over multiple visits to identify growth trends rather than relying on a single measurement.
• Attend all scheduled prenatal appointments and ultrasound scans for accurate monitoring.
• Maintain a balanced diet rich in protein, iron, calcium, and folate to support healthy fetal growth.
• Stay hydrated - adequate water intake supports amniotic fluid levels and nutrient delivery.
• Report any concerns about decreased fetal movement to your healthcare provider immediately.
• Remember that estimated weights have a ±15% margin of error - a range is more meaningful than a single number.
• Ask your healthcare provider to explain the growth percentile trend at each visit.
• Avoid comparing your baby's size to others - healthy babies come in many sizes.

Frequently Asked Questions