Estimate fetal weight and project birth size with professional accuracy.
Enter current weeks (20-42).
Please enter a value between 20 and 42.
0 days
1 day
2 days
3 days
4 days
5 days
6 days
Additional days past the last full week.
Unknown / Average
Boy (Tend to be heavier)
Girl (Tend to be lighter)
Adjusts calculation based on gender growth patterns.
Average Build
Petite / Low BMI
Larger Build / High BMI
Maternal size can correlate with fetal size.
Estimated Current Fetal Weight
1,319 g
(approx. 2 lbs 14 oz)
Projected Birth Weight (40w):3,500 g
Normal Range (10th-90th %):1,100g – 1,500g
Growth Percentile Estimate:50th Percentile
Weeks Remaining:10 weeks
Formula Note: This predict birth weight calculator uses a polynomial regression based on standard WHO and NICHD fetal growth charts, adjusted for the entered variables.
Fetal Growth Trajectory
Blue Line: Average Growth | Red Dot: Your Current Estimate
Estimated Weight Milestones
Gestational Age
10th Percentile
50th Percentile (Avg)
90th Percentile
Comprehensive Guide to Using the Predict Birth Weight Calculator
Monitoring fetal growth is a central part of prenatal care. Using a predict birth weight calculator allows expectant parents and medical professionals to estimate the size of a baby at various stages of pregnancy and project their size at full term. This tool helps in preparing for delivery and understanding the developmental trajectory of the fetus.
A predict birth weight calculator is a digital tool designed to estimate the Estimated Fetal Weight (EFW) based on clinical variables such as gestational age, fetal gender, and maternal characteristics. Unlike a simple timeline, a robust calculator adjusts for biological variations to provide a personalized range.
While ultrasounds provide the most accurate measurements (using metrics like Abdominal Circumference and Femur Length), this calculator provides a statistical estimate based on population growth standards. It is useful for identifying if a fetus is Small for Gestational Age (SGA) or Large for Gestational Age (LGA), both of which can influence birth plans.
Predict Birth Weight Calculator Formula
The logic behind a predict birth weight calculator typically involves regression analysis derived from large population studies (such as the Hadlock or Intergrowth-21st standards). The baseline growth follows a curve that accelerates in the third trimester.
Our calculator uses a polynomial approximation of the 50th percentile growth curve, modified by coefficients for gender and maternal build.
Variables Used in Calculation
Variable
Definition
Impact on Result
Gestational Age (GA)
Time elapsed since the last menstrual period (Weeks + Days).
Primary driver of weight calculation.
Gender Coefficient
Biological sex of the fetus.
Males are typically ~3-5% heavier than females at term.
Maternal Build
Mother's pre-pregnancy BMI category.
Higher maternal weight correlates with higher fetal weight.
Growth Velocity
Rate of weight gain per week.
Increases from ~150g/week at 28 weeks to ~200g+/week at 36 weeks.
Practical Examples
To better understand how the predict birth weight calculator functions, consider these real-world scenarios.
Calculation: Baseline for 38.5 weeks is roughly 3,100g. The "Boy" factor (+3%) and "Large Build" factor (+2%) increase the estimate.
Result: Estimated current weight is ~3,250g. The projection for 40 weeks suggests a larger baby, potentially exceeding 3,800g (8 lbs 6 oz).
How to Use This Predict Birth Weight Calculator
Follow these simple steps to get an accurate estimation:
Enter Gestational Age: Input the current number of completed weeks and days. This is the most critical factor.
Select Gender: If known, choose Boy or Girl. If unknown, leave as "Unknown/Average" for a blended result.
Select Maternal Build: Choose the option that best describes the mother's pre-pregnancy size.
Review Results: Look at the "Estimated Current Fetal Weight" and the "Projected Birth Weight".
Check the Chart: Use the visual graph to see where the estimate falls relative to the standard growth curve.
Key Factors That Affect Predict Birth Weight Results
Several biological and environmental factors influence the final output of a predict birth weight calculator:
Genetics: Parental height and weight are strong predictors. Tall parents often have longer, heavier babies.
Placental Health: The placenta's ability to transfer nutrients dictates growth. Insufficiency can lead to lower weights.
Gestational Diabetes: Uncontrolled sugars often result in macrosomia (large babies), causing weights significantly above the 90th percentile.
Multiples: Twins or triplets usually grow more slowly per fetus in the third trimester compared to singletons.
Nutrition: Adequate maternal protein and caloric intake supports optimal growth.
Smoking/Health Habits: Smoking is strongly correlated with lower birth weights due to restricted oxygen flow.
Frequently Asked Questions (FAQ)
How accurate is this predict birth weight calculator?
Clinical estimates have a margin of error of about +/- 15%. This web-based predict birth weight calculator provides statistical averages. Ultrasound estimation is generally more accurate but still estimates weight within a 10-20% range.
What is considered a normal birth weight?
A term baby (37-42 weeks) typically weighs between 2,500g (5 lbs 8 oz) and 4,000g (8 lbs 13 oz). Anything below 2,500g is considered Low Birth Weight (LBW).
Can I change my baby's birth weight?
While genetics play a huge role, maintaining a healthy diet, managing blood sugar levels, and avoiding smoking can help ensure your baby reaches a healthy weight.
Does the calculator account for twins?
This calculator is calibrated for singleton pregnancies. Twin growth curves diverge significantly after week 28-30.
Why is the projected weight at 40 weeks different from my doctor's estimate?
Doctors may use ultrasound measurements (AC, FL, BPD) which reflect actual physical size, whereas this calculator uses gestational age averages. Always defer to your medical provider.
Does a large estimated weight mean I need a C-section?
Not necessarily. Measurements are estimates, and many women deliver larger babies vaginally without issue. Discuss your specific measurements with your OB-GYN.
What if my result is in the 10th percentile?
This may indicate the baby is smaller than average. It could be constitutionally small (healthy but small) or due to growth restriction. Medical monitoring is usually recommended.
How often should I use the predict birth weight calculator?
It is helpful to check weekly to track the trajectory, but remember that fetal growth can occur in spurts rather than a perfectly linear line.
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