Estimate your baby's weight based on common ultrasound measurements. Please consult with your healthcare provider for accurate medical advice.
Enter the number of weeks from the first day of your last menstrual period.
The widest diameter of the fetal head.
The diameter of the fetal head from front to back.
The circumference of the fetal abdomen.
The length of the fetal femur (thigh bone).
Estimated Fetal Weight (EFW)
—
Head Area (HA): —
Cephalic Index (CI): —
Estimated Weight (g): —
Formula Basis: This calculator uses established sonographic formulas, often derived from Hadlock or similar methods, which combine head and abdominal measurements to estimate fetal weight. The Head Area (HA) is calculated from BPD and OFD, and the Cephalic Index (CI) from BPD and OFD. These indices, along with AC and FL, are inputs into regression equations to estimate weight.
Fetal Growth Trend
Visualizing estimated fetal weight against typical growth curves.
Typical Fetal Weight by Gestational Age
Comparison of calculated EFW with average weights.
What is Fetal Weight Estimation?
Fetal weight estimation, often referred to as Estimated Fetal Weight (EFW), is a process used during pregnancy to approximate the weight of a fetus. This estimation is typically performed using non-invasive diagnostic imaging techniques, most commonly ultrasound. Healthcare providers use EFW to monitor fetal growth, assess potential complications like intrauterine growth restriction (IUGR) or macrosomia (excessively large baby), and guide decisions regarding delivery management. It's crucial to understand that EFW is an estimate and not a precise measurement; actual birth weight can vary.
Who should use it: Pregnant individuals undergoing prenatal care, especially those with specific concerns about fetal growth, multiple gestations, or if routine ultrasounds suggest a deviation from the expected growth curve. It is primarily a tool for healthcare providers but can offer insights to expectant parents curious about their baby's development.
Common misconceptions: A significant misconception is that EFW is perfectly accurate. It's an approximation with a margin of error that can increase as pregnancy progresses. Another is that EFW alone dictates delivery decisions; it's one factor among many, including maternal health, fetal well-being indicators, and gestational age.
Fetal Weight Estimation Formula and Mathematical Explanation
The calculation of Estimated Fetal Weight (EFW) is not based on a single, universally applied formula, but rather on a variety of regression equations derived from large datasets of ultrasound measurements and actual birth weights. These formulas aim to predict the baby's weight based on key biometric parameters measured via ultrasound. Common parameters include:
Biparietal Diameter (BPD): The transverse diameter of the fetal head.
Occipitofrontal Diameter (OFD): The diameter of the fetal head from the frontal bone to the occiput.
Head Circumference (HC): Calculated from BPD and OFD, or measured directly.
Abdominal Circumference (AC): A measure of the fetal abdomen.
Femur Length (FL): The length of the fetal femur.
The calculator above employs a simplified approach that first calculates intermediate values like Head Area (HA) and Cephalic Index (CI), which are then used in conjunction with AC and FL. A commonly cited simplified approach, for example, might use a formula similar to:
Where 'a', 'b', 'c', 'd', 'e' are coefficients determined by statistical analysis specific to the formula used (e.g., Hadlock, Shepard, Warsof). Our calculator uses a representative set of intermediate calculations and a conceptual formula structure.
Variables Table:
Variable
Meaning
Unit
Typical Range (Approx.)
Gestational Age
Age of pregnancy since LMP
Weeks
14 – 42
BPD
Biparietal Diameter
mm
20 – 95+
OFD
Occipitofrontal Diameter
mm
25 – 115+
AC
Abdominal Circumference
mm
150 – 350+
FL
Femur Length
mm
20 – 80+
HA (Head Area)
Calculated head cross-sectional area
cm²
Variable, derived from BPD/OFD
CI (Cephalic Index)
Ratio of BPD to OFD
% (or ratio)
70 – 85%
EFW
Estimated Fetal Weight
Grams / Kilograms
Variable, increases with GA
Practical Examples (Real-World Use Cases)
Example 1: Routine Mid-Pregnancy Scan
Scenario: A pregnant individual at 28 weeks of gestation undergoes a routine anatomy scan. The following measurements are obtained:
Gestational Age: 28 weeks
BPD: 70 mm
OFD: 85 mm
AC: 250 mm
FL: 52 mm
Calculator Input: Entering these values into the calculator.
Calculator Output:
Estimated Fetal Weight (EFW): Approximately 1050 grams (1.05 kg)
Head Area (HA): ~50 cm²
Cephalic Index (CI): ~82.35%
Estimated Weight (grams): ~1050 g
Interpretation: This EFW is generally within the normal range for 28 weeks of gestation. The CI is also normal, suggesting a typical head shape. This reassures the healthcare provider and the expectant parents that the baby's growth is on track.
Example 2: Concern for Growth Restriction
Scenario: A pregnant individual at 34 weeks of gestation has been monitored due to concerns about fetal growth. Ultrasound measurements show:
Gestational Age: 34 weeks
BPD: 78 mm
OFD: 95 mm
AC: 230 mm
FL: 65 mm
Calculator Input: Entering these values into the calculator.
Calculator Output:
Estimated Fetal Weight (EFW): Approximately 1700 grams (1.7 kg)
Head Area (HA): ~61.5 cm²
Cephalic Index (CI): ~82.1%
Estimated Weight (grams): ~1700 g
Interpretation: The estimated weight of 1700 grams at 34 weeks is below the typical average for this gestational age (which is usually closer to 2000-2200 grams). The abdominal circumference (AC) might be disproportionately smaller compared to head measurements. This finding could indicate Intrauterine Growth Restriction (IUGR) and would prompt further investigation, including Doppler studies and closer monitoring, to assess fetal well-being and decide on the best course of management.
How to Use This Fetal Weight Calculator
Our free online fetal weight calculator is designed to be simple and intuitive. Follow these steps for an estimated fetal weight (EFW):
Obtain Ultrasound Measurements: You will need specific measurements taken during an ultrasound examination. These typically include Gestational Age (in weeks), Biparietal Diameter (BPD), Occipitofrontal Diameter (OFD), Abdominal Circumference (AC), and Femur Length (FL). Ensure these measurements are in millimeters (mm).
Input Gestational Age: Enter the number of weeks of your pregnancy into the 'Gestational Age (Weeks)' field.
Input Biometric Data: Carefully enter the BPD, OFD, AC, and FL measurements into their respective fields, ensuring the units are correct (millimeters).
Perform Calculation: Click the 'Calculate' button.
View Results: The calculator will display the primary Estimated Fetal Weight (EFW) prominently, along with key intermediate values like Head Area (HA) and Cephalic Index (CI), and the estimated weight in grams.
How to Read Results: The primary EFW is an approximation of your baby's current weight. Compare this to standard fetal growth charts (which our chart visualization attempts to represent) for your specific gestational age. A result significantly above or below the average may warrant discussion with your healthcare provider.
Decision-Making Guidance: This calculator is for informational purposes only. It is NOT a substitute for professional medical diagnosis or advice. Any concerns arising from the EFW should be discussed immediately with your obstetrician or midwife. They will consider EFW alongside other factors like amniotic fluid levels, placental function, and fetal well-being tests.
Key Factors That Affect Fetal Weight Results
While the ultrasound biometry provides the input data, several underlying biological and external factors influence fetal growth and, consequently, the accuracy and interpretation of EFW estimates:
Gestational Age Accuracy: The accuracy of the EFW is highly dependent on the accuracy of the stated gestational age. Dating the pregnancy early using crown-rump length (CRL) is generally more precise than relying solely on later measurements.
Fetal Anatomy Variations: Some fetuses naturally have different body compositions (e.g., more muscle or fat). A baby with a larger abdominal circumference might weigh more, but this could be due to increased subcutaneous fat rather than overall growth.
Placental Function: The placenta's ability to deliver nutrients and oxygen is critical for fetal growth. Conditions like placental insufficiency can lead to restricted growth (IUGR), resulting in a lower EFW than expected for gestational age. Conversely, a very efficient placenta might contribute to a larger EFW.
Maternal Health Conditions: Maternal factors such as diabetes can lead to macrosomia (excessive fetal weight) due to increased glucose supply. Maternal hypertension or other vascular conditions can impair placental function and lead to growth restriction.
Genetic Factors: Inherited traits play a significant role. If parents are tall or have large body frames, their baby may naturally be larger. Similarly, genetic conditions can affect fetal growth rates.
Multiple Gestations: In twin or triplet pregnancies, fetuses often share resources, which can lead to varying growth rates among siblings. One twin might be significantly smaller or larger than the other due to factors like placental sharing (e.g., Twin-to-Twin Transfusion Syndrome).
Amniotic Fluid Volume: Significantly high (polyhydramnios) or low (oligohydramnios) amniotic fluid levels can sometimes be associated with fetal growth abnormalities and can also affect the accuracy of ultrasound measurements.
Ultrasound Equipment and Operator Skill: The quality of the ultrasound machine and the experience of the sonographer performing the measurements can introduce variability. Precise placement of cursors is essential for accurate BPD, OFD, AC, and FL measurements.
Frequently Asked Questions (FAQ)
Q1: Is the estimated fetal weight (EFW) the baby's actual birth weight?
A: No, EFW is an estimate. Ultrasound measurements have a margin of error, typically around 10-15%. The actual birth weight can differ.
Q2: When is fetal weight estimated?
A: Fetal weight can be estimated throughout pregnancy using ultrasound, but it becomes more clinically relevant from the second trimester onwards. Major growth assessments are often done during the anatomy scan (around 18-22 weeks) and in the third trimester.
Q3: What is considered a normal fetal weight at 30 weeks?
A: At 30 weeks, a typical range for fetal weight is roughly 1300-1600 grams (1.3-1.6 kg). However, this is an average, and variations are common. Your doctor will assess your specific situation.
Q4: What does it mean if my baby's EFW is consistently low?
A: Consistently low EFW may indicate Intrauterine Growth Restriction (IUGR). This requires further evaluation to determine the cause (e.g., placental issues, maternal health) and ensure the baby is receiving adequate nutrients and oxygen. Management might involve closer monitoring or early delivery.
Q5: What does it mean if my baby's EFW is consistently high?
A: Consistently high EFW may suggest macrosomia or a large-for-gestational-age baby. This is often seen in pregnancies complicated by maternal diabetes. It can increase risks during labor and delivery (e.g., shoulder dystocia).
Q6: Can I use this calculator at home without an ultrasound?
A: No, this calculator requires specific biometric measurements (BPD, OFD, AC, FL) obtained from an ultrasound. It cannot provide an estimate based on guesswork or non-ultrasound methods.
Q7: How often are fetal weight estimates performed?
A: Routine growth scans might be done once or twice during pregnancy. More frequent estimations are performed if there are specific concerns about fetal growth, maternal health complications, or if previous assessments indicated potential issues.
Q8: Can the calculator predict birth weight?
A: It provides an estimate of the current fetal weight. While it can offer an indication, it's not a precise predictor of exact birth weight, especially as many factors influence the final weeks of growth.