Estimate fetal weight and track growth percentiles accurately
Current week of pregnancy (20-42 weeks).
0 Days
1 Day
2 Days
3 Days
4 Days
5 Days
6 Days
Additional days past the completed week.
Unknown / Average
Male (Boy)
Female (Girl)
Boys tend to be slightly heavier than girls.
Average Build
Petite / Small Frame
Large Frame / Tall
Maternal size often correlates with fetal size.
Estimated Fetal Weight
0 g
0 lbs 0 oz
Growth Percentile
50th
Est. Weight at 40 Weeks
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Weekly Weight Gain
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Calculation based on Hadlock fetal growth standards adjusted for gender and maternal factors.
Week
10th Percentile (g)
50th Percentile (g)
90th Percentile (g)
What is a Predicted Birth Weight Calculator?
A predicted birth weight calculator is a specialized tool designed to estimate the weight of a fetus at a specific point in pregnancy or predict the final birth weight at term (40 weeks). Unlike generic weight charts, a robust calculator considers variables such as gestational age, fetal gender, and maternal characteristics to provide a more personalized estimation.
Monitoring fetal weight is a critical component of prenatal care. It helps healthcare providers identify potential growth issues such as Macrosomia (large for gestational age) or Intrauterine Growth Restriction (IUGR). While ultrasound measurements (biometry) provide the most accurate data, mathematical models based on gestational age can offer parents a helpful baseline for understanding their baby's development trajectory.
This tool is ideal for expectant parents who want to track their baby's growth progress between doctor visits or understand the implications of their current gestational week on the baby's size.
Predicted Birth Weight Formula and Mathematical Explanation
Fetal weight estimation relies on statistical regression models derived from large population studies. The most widely accepted method in obstetrics is the Hadlock Formula, though simplified versions are used for age-based estimation without ultrasound metrics.
The core growth curve follows a polynomial function where weight increases non-linearly—slowly in the second trimester and rapidly in the third.
Key Variables in Estimation
Variable
Meaning
Impact on Result
Gestational Age (GA)
Time elapsed since the last menstrual period.
Primary driver of weight calculation.
Fetal Gender
Biological sex of the baby.
Males are typically 3-5% heavier than females at term.
Maternal Parity
Number of previous births.
Second and subsequent babies are often heavier.
Maternal BMI/Height
Mother's physical stature.
Taller/heavier mothers statistically have larger babies.
The Mathematical Model:
For a standard estimation based on weeks, we use the 50th percentile median weight ($W_{50}$) for the given week ($t$).
$$ W_{est} = W_{50}(t) \times M_{gender} \times M_{maternal} $$
Where $M_{gender}$ is 1.015 for males and 0.985 for females, and $M_{maternal}$ adjusts for genetic potential.
Practical Examples (Real-World Use Cases)
Example 1: The Standard Growth Track
Scenario: Sarah is 32 weeks pregnant with a boy. She has an average build.
Input: 32 Weeks, 0 Days.
Gender: Male.
Maternal Build: Average.
Calculation: The median weight for 32 weeks is approximately 1,702g. The male gender modifier adds ~1.5%.
Result: ~1,727g (3 lbs 13 oz).
Interpretation: The baby is tracking normally. The calculator predicts a term weight of roughly 3.5kg (7 lbs 11 oz) if growth continues on this percentile.
Example 2: Monitoring Potential Macrosomia
Scenario: Emily is 36 weeks pregnant. She is tall (Large frame) and expecting a girl.
Input: 36 Weeks, 0 Days.
Gender: Female.
Maternal Build: Large.
Calculation: Median weight for 36 weeks is ~2,620g. Female modifier reduces this slightly, but the Large Frame modifier increases it by ~5%.
Result: ~2,710g (5 lbs 15 oz).
Interpretation: Despite being a girl, the baby is tracking above the 50th percentile due to maternal genetics. This helps Emily prepare for potentially needing larger newborn clothes.
How to Use This Predicted Birth Weight Calculator
Select Gestational Age: Choose your current week and day of pregnancy. This is the most important input.
Input Gender: If you know the sex of the baby, select it. If not, leave as "Unknown" for a neutral estimate.
Select Maternal Build: Choose the option that best describes the mother's pre-pregnancy frame. This helps adjust for genetic factors.
Review Results:
Estimated Weight: The current predicted weight in grams and pounds/ounces.
Percentile: Where your baby falls compared to the average population.
Term Projection: An estimate of how big the baby might be at 40 weeks.
Analyze the Chart: Look at the graph to see the growth curve. The blue line represents the average; see where your estimate sits relative to the curve.
Key Factors That Affect Predicted Birth Weight Results
Several biological and environmental factors influence the final birth weight. Understanding these can help interpret the results of the predicted birth weight calculator.
1. Gestational Age
Time is the biggest factor. A baby gains approximately 200-250 grams per week in the third trimester. Even a difference of a few days can significantly alter the weight estimation.
2. Genetics and Ethnicity
Parental height and weight play a major role. Tall parents tend to have longer, heavier babies. Additionally, statistical averages vary slightly between different ethnic groups due to genetic variations in growth potential.
3. Maternal Health and Nutrition
Conditions like Gestational Diabetes can lead to larger babies (Macrosomia), while hypertension or preeclampsia may restrict blood flow to the placenta, leading to lower birth weights. Adequate protein and caloric intake are vital for optimal growth.
4. Placental Function
The placenta is the baby's life support system. If the placenta is functioning optimally, the baby will likely reach their genetic growth potential. Placental insufficiency is a primary cause of growth restriction.
5. Fetal Gender
On average, male fetuses grow faster than female fetuses, particularly in the late third trimester. This difference is usually small (approx. 100-150g at term) but statistically significant.
6. Multiples (Twins/Triplets)
Twins and multiples often have lower individual birth weights compared to singletons because they share the uterine space and resources. This calculator is calibrated for singleton pregnancies.
Frequently Asked Questions (FAQ)
How accurate are fetal weight estimates?
Estimates based on gestational age are statistical averages. Ultrasound estimates (using measurements like AC and FL) are generally accurate within +/- 15%. No calculator can be 100% precise due to variations in tissue density and fluid levels.
What is considered a "normal" birth weight?
A normal birth weight for a full-term baby (37-40 weeks) typically ranges from 2,500g (5 lbs 8 oz) to 4,000g (8 lbs 13 oz).
Does a high predicted weight mean I will have a C-section?
Not necessarily. While suspected Macrosomia (large baby) is a factor doctors consider, the pelvis size and labor progression are equally important. A predicted weight is just one piece of the puzzle.
Why is my baby measuring small/large?
Discrepancies can be due to incorrect dating (wrong due date), genetics (petite parents), or medical factors. Always consult your OB-GYN if you are concerned about growth percentiles.
Can I influence my baby's birth weight?
Maintaining a healthy diet, managing blood sugar levels, and avoiding smoking can help ensure your baby reaches a healthy weight. However, genetic factors are largely beyond control.
Does the calculator work for twins?
This predicted birth weight calculator is designed for singleton pregnancies. Twins have different growth curves, typically slowing down after week 30 compared to singletons.
What is the 90th percentile?
If a baby is in the 90th percentile, it means they weigh more than 90% of other babies at the same gestational age. This is often an indicator of a larger-than-average baby.
When is the most rapid weight gain?
The most rapid weight gain occurs between weeks 32 and 36, where a fetus can gain over half a pound per week as they lay down fat stores for temperature regulation outside the womb.