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Estimated Birth Weight Calculator

Estimate your baby's birth weight based on maternal factors, gestational age, and ultrasound measurements. This tool helps healthcare providers and expectant parents anticipate fetal growth.

Weeks of pregnancy (between 20-42 weeks)
Height in centimeters
Pre-pregnancy weight in kilograms
Age in years
Number of previous births (not including current pregnancy)
Ultrasound measurement in centimeters
Ultrasound measurement in centimeters

How to Use This Calculator

  1. Enter the gestational age in weeks (from 20 to 42 weeks)
  2. Input maternal characteristics including height, pre-pregnancy weight, and age
  3. Provide parity (number of previous births) and fetal sex if known
  4. Enter ultrasound measurements including biparietal diameter and abdominal circumference
  5. Click Calculate to see the estimated birth weight

Formula Used

EBW = (GA × 95) + (BPD × 135) + (AC × 45) + (MH × 8.5) + (MW × 12) + (PA × 25) + (S × 60) - 4230

Where:

  • EBW = Estimated Birth Weight (grams)
  • GA = Gestational Age (weeks)
  • BPD = Biparietal Diameter (cm)
  • AC = Abdominal Circumference (cm)
  • MH = Maternal Height (cm)
  • MW = Maternal Weight (kg)
  • PA = Parity (number of previous births)
  • S = Sex factor (Male = 1, Female = 0, Unknown = 0.5)

Example Calculation

Real-World Scenario:

A 32-year-old pregnant woman at 38 weeks gestation is expecting her second child. She is 165 cm tall and weighed 58 kg before pregnancy. Ultrasound shows a female fetus with BPD of 9.2 cm and AC of 33.5 cm.

Given:

  • Gestational Age = 38 weeks
  • Maternal Height = 165 cm
  • Maternal Weight = 58 kg
  • Maternal Age = 32 years
  • Parity = 1 (one previous birth)
  • Fetal Sex = Female
  • Biparietal Diameter = 9.2 cm
  • Abdominal Circumference = 33.5 cm

Calculation:

EBW = (38 × 95) + (9.2 × 135) + (33.5 × 45) + (165 × 8.5) + (58 × 12) + (1 × 25) + (0 × 60) - 4230

EBW = 3610 + 1242 + 1507.5 + 1402.5 + 696 + 25 + 0 - 4230

Result: 4253 grams (approximately 4.25 kg or 9 lbs 6 oz)

Why This Calculation Matters

Practical Applications

  • Helps healthcare providers plan for delivery
  • Identifies potential risks for macrosomia or low birth weight
  • Guides decisions about delivery methods
  • Assists in neonatal care preparation

Key Benefits

  • Provides expectant parents with information about their baby's development
  • Enables early intervention for potential growth issues
  • Reduces uncertainty during pregnancy
  • Supports personalized prenatal care

Common Mistakes & Tips

Using an incorrect gestational age is one of the most common errors. Always use the first-trimester ultrasound for the most accurate dating. If you're unsure about your due date, consult with your healthcare provider to confirm the gestational age before using this calculator.

Ultrasound measurements should be taken by trained professionals using standardized techniques. Ensure measurements are taken in the correct plane and units (centimeters). If possible, use the average of multiple measurements taken during the same ultrasound session for greater accuracy.

This calculator may not be accurate for pregnancies with special conditions such as multiple gestation, significant maternal medical conditions (like diabetes), or known fetal abnormalities. If you have any of these conditions, consult with your healthcare provider for a more personalized assessment of fetal growth.

Frequently Asked Questions

The accuracy of estimated birth weight calculations varies but is typically within ±10-15% of the actual birth weight when performed by experienced professionals. Accuracy tends to be higher in the third trimester and decreases with extremes of fetal size (very small or very large babies). Remember that this is an estimate, not a definitive prediction.

Birth weight estimates become more accurate as pregnancy progresses. While estimates can be made from around 20 weeks, they are most reliable in the third trimester, particularly after 34 weeks. However, even late-pregnancy estimates have a margin of error, and actual birth weight can vary significantly from predictions.

Many factors can influence fetal growth and birth weight, including genetics, maternal nutrition, maternal health conditions (like diabetes or hypertension), maternal weight gain during pregnancy, placental function, environmental factors, and gestational age at delivery. Male infants typically weigh slightly more than female infants, and birth weight generally increases with parity (number of previous births).

Normal birth weight typically ranges from 2,500 to 4,000 grams (5.5 to 8.8 pounds). Babies weighing less than 2,500 grams are considered low birth weight, and those weighing more than 4,000 grams are considered macrosomic (large for gestational age). These definitions may vary slightly based on gestational age and population characteristics.

References & Disclaimer

Medical Disclaimer

This calculator provides an estimate of fetal birth weight based on mathematical formulas and should not replace professional medical advice. The results are not diagnostic and should be interpreted by qualified healthcare providers in the context of a complete clinical evaluation. Always consult with your healthcare provider regarding any concerns about your pregnancy or fetal development.

References

Accuracy Notice

This calculator uses established formulas but has inherent limitations. The accuracy of birth weight estimation can be affected by factors such as fetal position, maternal body habitus, operator experience, and biological variation. The margin of error for estimated fetal weight is typically ±10-15% of actual birth weight. This calculator is not validated for use in multiple gestations or pregnancies with significant fetal abnormalities.

About the Author

Kumaravel Madhavan

Web developer and data researcher creating accurate, easy-to-use calculators across health, finance, education, and construction and more. Works with subject-matter experts to ensure formulas meet trusted standards like WHO, NIH, and ISO.

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