Enter the following ultrasound measurements to estimate your baby's weight. This calculator uses standard formulas for prediction.
Enter the number of full weeks of pregnancy.
Measured across the baby's head.
Measured around the baby's head.
Measured around the baby's abdomen.
Length of the baby's thigh bone.
Your Estimated Fetal Weight
—
Estimated Gestational Age: — weeks
Estimated Fetal Volume (EFW): — grams
Standard Deviation Score (SDS): —
Key Assumption: Measurements are accurate and taken using a calibrated ultrasound machine.
The primary estimation is often based on a combination of biometric measurements using established regression formulas, such as Hadlock's or others. A common approach involves calculating the Estimated Fetal Weight (EFW) using parameters like BPD, HC, AC, and FL. The Standard Deviation Score (SDS) is then calculated by comparing the EFW to the mean EFW for the gestational age and dividing by the standard deviation for that age.
Chart showing estimated fetal weight progression based on gestational age.
Estimated Fetal Weight and Percentiles by Gestational Age
Gestational Age (Weeks)
Mean Weight (g)
10th Percentile (g)
50th Percentile (g)
90th Percentile (g)
Your Estimated Weight (g)
What is a Fetal Weight Predictor Calculator?
A Fetal Weight Predictor Calculator is a tool designed to estimate the weight of a fetus during pregnancy. It utilizes measurements obtained from prenatal ultrasounds, such as the Biparietal Diameter (BPD), Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL), along with the gestational age of the pregnancy. This calculator helps healthcare providers and expectant parents gain insights into the baby's growth trajectory and identify potential concerns like fetal growth restriction (FGR) or macrosomia (excessively large baby). It's important to remember that these are estimations, and the actual birth weight may vary.
Who should use it: This calculator is primarily intended for use by healthcare professionals (obstetricians, sonographers) as a supplementary tool in prenatal care. Expectant parents may also use it to better understand ultrasound reports and discuss growth patterns with their doctors. It is not a diagnostic tool on its own but rather an aid in assessing fetal well-being.
Common misconceptions: A frequent misunderstanding is that the calculator provides an exact birth weight. In reality, it offers an estimate with a margin of error. Another misconception is that a single reading can definitively diagnose growth issues; serial measurements and clinical context are crucial for accurate assessment. The calculator's accuracy is also dependent on the precision of the ultrasound measurements and the specific formula used.
Fetal Weight Predictor Calculator Formula and Mathematical Explanation
The calculation of fetal weight from ultrasound measurements is complex and relies on statistical models derived from numerous studies. There isn't a single universal formula; several variations exist, with the Hadlock formulas being among the most widely used. These formulas use multiple regression analysis to predict fetal weight based on a combination of biometric parameters.
A generalized approach often involves these steps:
Input Measurements: Collect key fetal measurements from the ultrasound: Gestational Age (GA), Biparietal Diameter (BPD), Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL).
Apply Regression Formula: Plug these measurements into a chosen regression formula to calculate the Estimated Fetal Weight (EFW). For instance, a simplified representation of a Hadlock formula might look like:
EFW (grams) = exp(a + b*ln(BPD) + c*ln(HC) + d*ln(AC) + e*ln(FL) + ... + f(GA))
(Note: The actual coefficients 'a' through 'f' and the exact inclusion/weighting of variables depend on the specific formula and gestational age range).
Calculate Standard Deviation Score (SDS): Once EFW is obtained, it's compared to the average EFW for that specific gestational age. The SDS is calculated as:
SDS = (Actual EFW - Mean EFW for GA) / Standard Deviation for GA
This score helps contextualize the estimated weight relative to norms. A positive SDS means the baby is estimated to be larger than average, while a negative SDS means smaller than average.
Variables Table
Variables Used in Fetal Weight Estimation
Variable
Meaning
Unit
Typical Range (Approximate)
Gestational Age (GA)
Number of weeks and days since the first day of the last menstrual period.
Weeks
14 – 42
Biparietal Diameter (BPD)
Transverse diameter of the fetal head at the level of the thalami.
cm
2 – 9.5+
Head Circumference (HC)
Circumference of the fetal head measured at the same plane as BPD.
cm
12 – 37+
Abdominal Circumference (AC)
Transverse circumference of the fetal abdomen, typically measured at the level of the umbilical vein junction.
cm
10 – 36+
Femur Length (FL)
Length of the diaphysis (shaft) of the femur bone.
cm
1.5 – 8+
EFW
Estimated Fetal Weight.
grams (g)
Varies widely with GA
SDS
Standard Deviation Score. Indicates how many standard deviations the EFW is from the mean for the GA.
Score
-3 to +3 (typically monitored)
Practical Examples (Real-World Use Cases)
Example 1: Routine Check-up at 30 Weeks
Scenario: A pregnant individual at 30 weeks and 4 days gestation undergoes a routine ultrasound. The measurements are:
Interpretation: The estimated fetal weight of 1450g falls within the normal range for 30 weeks gestation, indicated by an SDS of -0.5, which is close to the average. This suggests the baby is growing as expected.
Example 2: Concern for Fetal Growth Restriction
Scenario: A healthcare provider is monitoring a pregnancy where there's concern about the baby's growth. An ultrasound at 34 weeks shows:
Interpretation: The estimated fetal weight of 1700g is lower than average for 34 weeks gestation, as reflected by the SDS of -2.1. This finding, especially if accompanied by other clinical indicators (like decreased amniotic fluid or abnormal Doppler studies), may support a diagnosis of Fetal Growth Restriction (FGR). Further monitoring and management decisions would be based on these findings in conjunction with the overall clinical picture.
How to Use This Fetal Weight Predictor Calculator
Using the Fetal Weight Predictor Calculator is straightforward, but accuracy depends on precise input. Follow these steps:
Gather Ultrasound Data: Obtain the latest ultrasound report containing the key fetal measurements: Gestational Age (in weeks), Biparietal Diameter (BPD in cm), Head Circumference (HC in cm), Abdominal Circumference (AC in cm), and Femur Length (FL in cm).
Enter Gestational Age: Input the number of full weeks of pregnancy into the 'Gestational Age (Weeks)' field.
Enter Biometric Measurements: Carefully enter the BPD, HC, AC, and FL measurements in centimeters into their respective fields. Ensure you are using the values from the most recent and reliable ultrasound.
Click Calculate: Press the 'Calculate Fetal Weight' button. The calculator will process the inputs using established formulas.
Review Results: The calculator will display:
Estimated Fetal Weight: The primary result, shown in grams and an approximate pound conversion.
Estimated Gestational Age: Confirms the age used in calculation.
EFW (Estimated Fetal Weight): The precise calculated weight in grams.
SDS (Standard Deviation Score): Indicates how the EFW compares to the average for that gestational age.
Interpret Findings: Compare the SDS to typical ranges. An SDS between -2 and +2 is generally considered within the normal growth curve. Scores outside this range might warrant further discussion with a healthcare provider.
Use Table and Chart: The table provides percentile comparisons for reference, and the chart visually represents expected growth.
Reset or Copy: Use the 'Reset' button to clear fields and start over. Use 'Copy Results' to save or share the key outputs.
Decision-Making Guidance: This tool is for informational purposes. High or low estimated weights, or significant deviations from the expected growth curve (indicated by SDS), should always be discussed with your obstetrician or midwife. They will consider these estimates alongside other clinical factors (maternal health, amniotic fluid levels, Doppler studies) to make informed decisions about your prenatal care.
Key Factors That Affect Fetal Weight Results
While the calculator uses standard formulas, several factors can influence the accuracy of the estimated fetal weight and the baby's actual growth:
Accuracy of Ultrasound Measurements: This is paramount. Fetal growth formulas are highly sensitive to small variations in BPD, HC, AC, and FL. Operator skill, equipment calibration, and fetal positioning can all impact measurement precision. Even a few millimeters difference can alter the EFW estimate.
Gestational Age Accuracy: The accuracy of the gestational age, especially if determined by the first-trimester ultrasound, is critical. If the GA is off by even a week, the "normal" weight range shifts, affecting the interpretation of the EFW.
Individual Fetal Growth Variation: Every baby grows at its own pace. Genetics play a significant role; babies often resemble the size of their parents. The formulas provide an average, but individual variations are normal.
Placental Function: The placenta is the baby's lifeline, providing nutrients and oxygen. If placental function is compromised (e.g., due to conditions like preeclampsia or placental insufficiency), nutrient transfer may be reduced, leading to slower fetal growth (FGR).
Maternal Health Conditions: Conditions such as diabetes (which can cause macrosomia), hypertension, chronic infections, or poor nutrition in the mother can significantly affect fetal growth patterns.
Amniotic Fluid Volume: The amount of amniotic fluid surrounding the baby can indirectly indicate growth status. Significantly low fluid (oligohydramnios) may be associated with FGR, while high fluid (polyhydramnios) can sometimes accompany macrosomia or other conditions.
Fetal Anatomy and Chromosomal Abnormalities: Certain fetal abnormalities or chromosomal conditions (like Down syndrome) can be associated with specific growth patterns or growth restriction.
Multiple Gestations: In twin or triplet pregnancies, growth may differ from singletons due to factors like placental sharing, umbilical cord variations, and increased competition for resources. Growth charts for multiples are often used separately.
Frequently Asked Questions (FAQ)
Q1: How accurate is a fetal weight calculator?
A: Fetal weight calculators provide estimations, typically with a margin of error of about +/- 10-15%. Accuracy depends heavily on the quality of the ultrasound measurements, the formula used, and the expertise of the sonographer.
Q2: What is considered a normal estimated fetal weight?
A: "Normal" varies greatly with gestational age. A common benchmark is the 10th to 90th percentile for the specific week of pregnancy. The Standard Deviation Score (SDS) is often used, with values between -2 and +2 generally considered within the expected range.
Q3: Can this calculator predict my baby's exact birth weight?
A: No, it provides an estimate based on current measurements. The baby's weight can change significantly in the remaining weeks of pregnancy, and factors like labor and delivery can influence the final weight.
Q4: My baby's estimated weight is low. Should I be worried?
A: A low estimate doesn't automatically mean there's a problem. It depends on the Standard Deviation Score (SDS) and trends over time. Discuss any concerns about low estimated weight with your healthcare provider, who will assess your baby's overall growth pattern and well-being.
Q5: What is the difference between EFW and SDS?
A: EFW (Estimated Fetal Weight) is the calculated weight in grams. SDS (Standard Deviation Score) contextualizes that EFW by comparing it to the average weight for that gestational age. An SDS of 0 means the EFW is exactly average; a positive SDS means above average; a negative SDS means below average.
Q6: Are there different formulas for fetal weight prediction?
A: Yes, several formulas exist (e.g., Hadlock, Shepard, Intergrowth-21st). They differ in the specific measurements used, the statistical models applied, and the populations they were derived from. This calculator uses a common approach incorporating BPD, HC, AC, and FL.
Q7: Can I use this calculator at home without an ultrasound?
A: No, this calculator requires specific measurements (BPD, HC, AC, FL) obtained during an ultrasound examination. It is not designed for use with subjective estimations.
Q8: How often should fetal weight be monitored?
A: Monitoring frequency depends on individual circumstances. For uncomplicated pregnancies, it might be done once or twice during the third trimester. For high-risk pregnancies or concerns about growth, more frequent monitoring may be recommended by a healthcare provider.