Estimate your baby's expected weight based on their gestational age and parent factors.
Fetal Growth Estimator
Enter your gestational age and key maternal/paternal factors to estimate the baby's birth weight.
Enter the number of weeks since the last menstrual period.
Enter the mother's height in centimeters.
Enter the father's height in centimeters.
Enter the mother's weight before pregnancy in kilograms.
Enter the mother's age at the time of conception.
Caucasian
African
Asian
Hispanic
Other
Select the mother's ethnicity.
Estimated Birth Weight
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Avg. Weekly Gain (g)
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Estimated Length (cm)
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Growth Percentile
This estimate is based on a regression model incorporating gestational age, parental heights, maternal pre-pregnancy weight, maternal age, and ethnicity. It is a statistical approximation and not a definitive medical measurement.
Fetal Weight Progression Chart
Idealized Growth CurveYour Estimated Curve
Chart data is illustrative and based on the calculator's estimates.
Typical Fetal Weight by Gestational Week
Gestational Age (Weeks)
Estimated Fetal Weight (g)
Estimated Fetal Weight (lbs oz)
24
630
1 lb 6 oz
25
700
1 lb 9 oz
26
800
1 lb 12 oz
27
900
2 lb 0 oz
28
1000
2 lb 3 oz
29
1130
2 lb 8 oz
30
1300
2 lb 14 oz
31
1470
3 lb 4 oz
32
1640
3 lb 10 oz
33
1820
4 lb 0 oz
34
2000
4 lb 7 oz
35
2210
4 lb 14 oz
36
2420
5 lb 5 oz
37
2640
5 lb 13 oz
38
2850
6 lb 5 oz
39
3040
6 lb 11 oz
40
3200
7 lb 1 oz
41
3300
7 lb 4 oz
42
3350
7 lb 6 oz
What is a Birth Weight Calculator by Week?
A birth weight calculator by week is an online tool designed to provide an estimation of a baby's weight at different stages of gestation, typically expressed in grams or pounds and ounces. It leverages statistical models and data derived from numerous pregnancies to predict fetal growth based on factors such as the number of weeks pregnant (gestational age) and various maternal and paternal characteristics. This type of calculator is not a diagnostic tool but rather an informational resource for expectant parents and healthcare providers, offering insights into whether a baby's estimated weight falls within typical growth ranges for their gestational age.
Who should use it? Expectant parents curious about their baby's development, healthcare providers looking for a quick reference, or individuals interested in understanding fetal growth patterns. It's particularly useful for comparing estimated fetal weight against standard growth charts. Common misconceptions include believing the calculator provides an exact weight or that it can diagnose potential growth issues, which is not its intended purpose.
{primary_keyword} Formula and Mathematical Explanation
The calculation behind a birth weight calculator by week is complex, often relying on sophisticated regression models or algorithms derived from large-scale obstetric datasets. While the exact proprietary algorithms vary between calculators, a common approach involves a multi-variable linear regression. This method identifies the relationship between gestational age and a combination of other influencing factors to predict birth weight.
A simplified representation of the underlying principle can be expressed as:
Gestational Age (GA): The primary driver of fetal growth, representing the number of weeks of pregnancy.
Maternal Height (MH) & Paternal Height (PH): Parental stature is a significant indicator of potential baby size. Taller parents tend to have larger babies.
Maternal Pre-pregnancy Weight (MW): A mother's weight before conception often correlates with fetal growth potential, especially if she gains weight appropriately during pregnancy.
Maternal Age (MA): Age can influence fetal growth; very young or older mothers may have slightly different growth patterns.
Ethnicity Factor (E): Different ethnic groups can exhibit variations in average birth weight and growth trajectories. This is often represented by dummy variables or specific coefficients for each group.
Intercept & Coefficients: These are constants derived from statistical analysis of actual birth data. They determine the baseline weight and the impact of each input variable on the final estimate.
The accuracy of the estimate depends heavily on the quality and representativeness of the data used to train the model and the unique physiological factors of each pregnancy.
Variables Table
Variables Used in Birth Weight Estimation
Variable
Meaning
Unit
Typical Range
Gestational Age
Weeks of pregnancy
Weeks
20 – 42
Maternal Height
Mother's height
cm
140 – 200
Paternal Height
Father's height
cm
150 – 210
Maternal Pre-pregnancy Weight
Mother's weight before pregnancy
kg
40 – 150
Maternal Age
Mother's age at conception
Years
16 – 50
Maternal Ethnicity
Categorical variable for ethnicity
Category
Caucasian, African, Asian, Hispanic, Other
Estimated Birth Weight
Predicted weight of the baby at birth
Grams (g)
~500g – 4500g+
Estimated Length
Predicted length of the baby
Centimeters (cm)
~25cm – 55cm+
Average Weekly Gain
Average grams gained per week
g/week
~150g – 300g (later trimesters)
Growth Percentile
Estimated position relative to average growth
Percentile (%)
1 – 99
Practical Examples (Real-World Use Cases)
Understanding how the birth weight calculator by week works can be best illustrated with examples:
Example 1: First-Time Parents
Sarah and Ben are expecting their first child and are at 37 weeks of gestation. Sarah is 168 cm tall and weighed 58 kg before pregnancy. Ben is 185 cm tall. Sarah is 29 years old and identifies as Caucasian. They are eager to know how big their baby might be.
Inputs:
Gestational Age: 37 weeks
Maternal Height: 168 cm
Paternal Height: 185 cm
Maternal Pre-pregnancy Weight: 58 kg
Maternal Age: 29 years
Maternal Ethnicity: Caucasian
Calculator Output:
Estimated Birth Weight: 3150 g (approximately 6 lb 15 oz)
Average Weekly Gain: 200 g
Estimated Length: 49.5 cm
Growth Percentile: 55th percentile
Interpretation: The calculator suggests their baby is likely to be of average size for this gestational age, weighing around 7 pounds. This estimate is influenced by their relatively average heights and Sarah's pre-pregnancy weight. Being in the 55th percentile indicates their baby is estimated to be larger than 55% of babies at the same gestational age.
Example 2: Concerned Parents with Specific Factors
Maria and Carlos are at 34 weeks. Maria is 155 cm tall and weighed 70 kg before pregnancy. Carlos is 175 cm tall. Maria is 40 years old and identifies as Hispanic. They are concerned about potential complications given Maria's age and higher pre-pregnancy weight.
Inputs:
Gestational Age: 34 weeks
Maternal Height: 155 cm
Paternal Height: 175 cm
Maternal Pre-pregnancy Weight: 70 kg
Maternal Age: 40 years
Maternal Ethnicity: Hispanic
Calculator Output:
Estimated Birth Weight: 2400 g (approximately 5 lb 5 oz)
Average Weekly Gain: 190 g
Estimated Length: 45 cm
Growth Percentile: 30th percentile
Interpretation: The calculator estimates a lower-than-average weight for this gestational age, placing the baby in the 30th percentile. This might be influenced by a combination of factors, including maternal height, and potentially reflects a different growth trajectory for babies born to older mothers or those with specific ethnic backgrounds, though maternal pre-pregnancy weight can sometimes correlate with larger babies. This result could prompt a discussion with their doctor about potential causes and monitoring.
How to Use This Birth Weight Calculator by Week
Using our birth weight calculator by week is straightforward. Follow these steps to get your estimated fetal weight:
Enter Gestational Age: Input the number of weeks you are into your pregnancy. This is the most critical factor for determining fetal growth.
Input Parental Information: Provide the height (in cm) for both the mother and father.
Enter Maternal Health Data: Input the mother's pre-pregnancy weight (in kg) and her current age.
Select Ethnicity: Choose the mother's ethnicity from the dropdown menu, as this can influence growth patterns.
Calculate: Click the "Calculate" button.
How to Read Results:
The calculator will display:
Estimated Birth Weight: The primary output, given in grams and an approximate pounds/ounces conversion.
Average Weekly Gain: An estimate of how much weight the baby is gaining per week at this stage.
Estimated Length: A statistical prediction of the baby's length from head to heel.
Growth Percentile: Indicates how the baby's estimated weight compares to other babies at the same gestational age. A 50th percentile means the baby is estimated to be average.
Decision-Making Guidance: Remember, this calculator provides an *estimate*. It is not a substitute for medical advice or professional ultrasound measurements. If your results are significantly higher or lower than expected, or if you have concerns, discuss them with your obstetrician or midwife. They can provide accurate assessments and address any potential issues related to fetal growth.
Key Factors That Affect Birth Weight Results
Several elements can influence a baby's birth weight, and understanding these helps interpret the calculator's output:
Gestational Age: The most significant factor. Longer gestation generally leads to higher birth weight, up to a certain point. Premature babies naturally weigh less.
Genetics: Parental height and general build play a substantial role. If parents are tall or have a history of larger babies, the fetus is more likely to be larger.
Maternal Nutrition & Weight Gain: Adequate nutrition during pregnancy is crucial. A mother's pre-pregnancy weight and appropriate weight gain during pregnancy strongly correlate with fetal growth. Insufficient gain can lead to a smaller baby, while excessive gain might indicate a larger one or other issues.
Maternal Health Conditions: Conditions like diabetes (gestational or pre-existing) can lead to macrosomia (large baby), while others like placental insufficiency can restrict growth, resulting in a smaller baby (Intrauterine Growth Restriction – IUGR).
Maternal Age: Very young mothers (teenagers) and older mothers (over 35-40) can sometimes have babies with different birth weights compared to those in their mid-20s, though this is often mediated by other health factors.
Ethnicity: Studies show variations in average birth weights across different ethnic groups, potentially due to genetic predispositions, environmental factors, or socioeconomic influences.
Number of Fetuses: Twins, triplets, or other multiples typically weigh less individually than singletons at the same gestational age due to competition for resources.
Lifestyle Factors: Smoking, alcohol consumption, and drug use during pregnancy can significantly restrict fetal growth, leading to lower birth weights.
Frequently Asked Questions (FAQ)
Q1: How accurate is a birth weight calculator by week?
A: These calculators provide statistical estimates based on averages. They are not as accurate as medical ultrasounds or actual birth weight. Accuracy can vary, with estimates typically within a range of +/- 10-15% of the actual weight.
Q2: Can this calculator diagnose growth problems like IUGR or macrosomia?
A: No, this calculator is purely informational. It cannot diagnose Intrauterine Growth Restriction (IUGR) or macrosomia. If you have concerns about your baby's growth, consult your healthcare provider immediately.
Q3: Why is parental height important for the calculation?
A: Genetics play a significant role in a baby's potential size. Taller parents tend to have larger babies, and the calculator incorporates this genetic predisposition.
Q4: Does maternal weight gain during pregnancy affect the estimate?
A: While this calculator uses pre-pregnancy weight, actual weight gain during pregnancy is a crucial factor for fetal growth. Healthy weight gain supports optimal fetal development. Our calculator uses pre-pregnancy weight as a baseline indicator.
Q5: What does the growth percentile mean?
A: The percentile indicates how your baby's estimated weight compares to other babies of the same gestational age. For example, the 70th percentile means the baby is estimated to be heavier than 70% of babies at that age and lighter than 30%.
Q6: Can ethnicity really affect birth weight?
A: Yes, population studies show average differences in birth weights across various ethnic groups. These differences can be influenced by a complex interplay of genetic, environmental, and socioeconomic factors.
Q7: Is it better to be in a higher or lower percentile?
A: Neither extreme is inherently "better." Both very high birth weight (macrosomia) and very low birth weight (often associated with prematurity or IUGR) can carry risks. The goal is typically a healthy, appropriate-for-gestational-age weight. Discuss your percentile with your doctor.
Q8: How often should I recalculate my baby's estimated weight?
A: You can use the calculator anytime you want an updated estimate, especially as you progress through different weeks of pregnancy. However, frequent recalculation is usually unnecessary unless advised by a healthcare provider.