Estimate your baby's weight using key ultrasound measurements and gestational age.
Enter the number of weeks pregnant (e.g., 20.5 for 20 weeks and 3 days).
The widest diameter of the fetal head, measured from the outside edge of the near parietal bone to the inside edge of the far parietal bone.
The diameter of the fetal head from the front (occiput) to the back (frontal bone).
The circumference of the fetal head, measured at the same level as the BPD.
The circumference of the fetal abdomen, measured at the level of the liver and stomach.
The length of the fetal femur (thigh bone).
Estimated Foetal Weight (EFW)
— g
Estimated Fetal Weight (Hadlock):— g
Estimated Fetal Weight (Shepard):— g
Estimated Fetal Weight (Merks):— g
Estimated Fetal Age (by AC):— weeks
The Estimated Foetal Weight (EFW) is calculated using various established sonographic formulas that correlate ultrasound measurements like Biparietal Diameter (BPD), Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL) with fetal weight. These formulas are empirical and derived from studies on large populations of pregnant women. We display results from common models like Hadlock, Shepard, and Merks, acknowledging that different models may yield slightly different estimates. The AC is often used to estimate gestational age due to its strong correlation with fetal growth.
Chart: Estimated Foetal Weight vs. Gestational Age (based on input AC)
Common Foetal Growth Standards (approximate ranges)
Gestational Age (Weeks)
Estimated Foetal Weight (50th Percentile – g)
Estimated Foetal Weight (10th Percentile – g)
Estimated Foetal Weight (90th Percentile – g)
20
280
210
350
24
640
480
800
28
1060
790
1330
32
1650
1230
2070
36
2500
1850
3150
40
3500
2600
4400
Estimated Foetal Weight Calculator: Understanding Your Baby's Growth
The journey of pregnancy is filled with anticipation and a desire to understand every aspect of your developing baby. One of the key metrics healthcare providers monitor is the baby's estimated weight, often referred to as Estimated Foetal Weight (EFW). This is where an Estimated Foetal Weight Calculator becomes invaluable. By utilizing specific ultrasound measurements and the gestational age of the pregnancy, these calculators provide a data-driven estimate of your baby's size. This information can help in monitoring growth, planning for delivery, and ensuring the well-being of both mother and child.
What is an Estimated Foetal Weight Calculator?
An Estimated Foetal Weight Calculator is a specialized tool designed to predict the approximate weight of a fetus during pregnancy. It operates by taking several key biometric measurements obtained during an ultrasound scan and inputting them, along with the gestational age, into established mathematical formulas. These formulas have been developed and refined over decades by medical researchers to correlate physical dimensions with fetal mass.
Who should use it?
Expectant parents who are curious about their baby's size and growth trajectory.
Healthcare providers (obstetricians, sonographers, midwives) as a supplementary tool for monitoring fetal growth and identifying potential concerns like intrauterine growth restriction (IUGR) or macrosomia (a baby significantly larger than average).
Researchers studying fetal development and growth patterns.
Common Misconceptions:
"It's a perfect measurement." EFW is an estimate, not an exact weight. There's always a margin of error, typically around 10-15%.
"It determines the baby's final birth weight precisely." While it's a good indicator, factors can change rapidly in the final weeks, and the actual birth weight can differ.
"It's the only way to know if the baby is growing well." While important, EFW is just one piece of the puzzle. Amniotic fluid levels, placental health, and maternal well-being are also critical.
Estimated Foetal Weight Calculator Formula and Mathematical Explanation
The core of an Estimated Foetal Weight Calculator lies in the various sonographic formulas used. These are not single, universal formulas but rather a collection of models developed by different researchers, each with its own strengths and typical applications. The most common measurements used are:
Biparietal Diameter (BPD)
Head Circumference (HC)
Abdominal Circumference (AC)
Femur Length (FL)
These measurements are then plugged into formulas like the Hadlock, Shepard, or Merks models. The formulas often involve exponential or polynomial functions to best fit the observed data relating measurements to weight.
A simplified example of a formula (illustrative, not the exact ones used in complex calculators):
One common approach involves a logarithmic transformation and then exponentiation. A generalized form might look like:
Log(EFW) = a + b*Log(BPD) + c*Log(AC) + d*Log(FL)
Where 'a', 'b', 'c', and 'd' are constants determined by the specific formula and study population.
Variable Explanations:
The accuracy of the EFW estimate is heavily dependent on the accuracy of the measurements and the appropriateness of the formula for the specific gestational age and fetal constitution.
Variables Used in EFW Calculations
Variable
Meaning
Unit
Typical Range
Gestational Age
Age of pregnancy since LMP
Weeks
16-42 weeks
BPD
Biparietal Diameter (widest head measurement)
mm
20 – 90+
OFD
Occipitofrontal Diameter (head length)
mm
30 – 100+
HC
Head Circumference
mm
130 – 370+
AC
Abdominal Circumference
mm
120 – 400+
FL
Femur Length (thigh bone)
mm
20 – 80+
The Abdominal Circumference (AC) is particularly crucial as it's closely related to fetal liver size and subcutaneous fat, which are major contributors to fetal weight. Therefore, AC is also frequently used to estimate fetal age independently. The calculator may show an Estimated Foetal Age based on the AC measurement to provide a comparison.
Practical Examples (Real-World Use Cases)
Example 1: Standard Growth Monitoring
A pregnant individual at 30 weeks and 4 days (30.6 weeks) has an ultrasound. The measurements are:
BPD: 78 mm
OFD: 90 mm
HC: 285 mm
AC: 270 mm
FL: 57 mm
When entered into our Estimated Foetal Weight Calculator:
Primary Result (e.g., Hadlock EFW): Approximately 1450 grams (1.45 kg)
Intermediate Value (AC Estimated Age): Around 30 weeks
Intermediate Value (Shepard EFW): Approximately 1380 grams
Intermediate Value (Merks EFW): Approximately 1520 grams
Interpretation: The baby's weight is estimated to be around 1450 grams. The AC measurement confirms the gestational age is consistent with the dating scan. This weight falls within the expected range for 30-31 weeks of gestation, suggesting normal growth. The slight variations between models highlight the estimated nature of these figures.
Example 2: Suspected Growth Deviation
A pregnant individual at 34 weeks gestation has an ultrasound due to concerns about the baby being small. The measurements are:
BPD: 80 mm
OFD: 95 mm
HC: 280 mm
AC: 230 mm
FL: 62 mm
When entered into our Estimated Foetal Weight Calculator:
Primary Result (e.g., Hadlock EFW): Approximately 1700 grams (1.7 kg)
Intermediate Value (AC Estimated Age): Around 31 weeks
Intermediate Value (Shepard EFW): Approximately 1650 grams
Intermediate Value (Merks EFW): Approximately 1780 grams
Interpretation: The baby's estimated weight is around 1700 grams. However, the AC measurement suggests the fetal age based on abdominal size is closer to 31 weeks, significantly lagging behind the expected 34 weeks. The EFW of 1700g is also on the lower end for 34 weeks (typically around 2100-2200g). This discrepancy warrants further investigation by the healthcare provider to assess fetal well-being and potential causes for slowed growth, such as placental insufficiency. This highlights how an Estimated Foetal Weight Calculator can flag potential issues.
How to Use This Estimated Foetal Weight Calculator
Using our calculator is straightforward and designed for clarity:
Gather Your Measurements: You will need the specific biometric measurements from your recent ultrasound report: Gestational Age (in weeks), Biparietal Diameter (BPD), Occipitofrontal Diameter (OFD), Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL). Ensure these are in millimeters (mm) unless specified otherwise.
Enter Gestational Age: Input the number of weeks of your pregnancy. You can use decimals for partial weeks (e.g., 28.3 weeks).
Input Biometric Data: Carefully enter each of the ultrasound measurements (BPD, OFD, HC, AC, FL) into the corresponding fields. Double-check the units (usually mm).
Calculate: Click the "Calculate Weight" button.
Read the Results: The calculator will display:
Primary Estimated Foetal Weight (EFW): The main highlighted result, usually based on a widely accepted formula like Hadlock.
Intermediate Values: Estimates from other common formulas (Shepard, Merks) and an estimated fetal age based on AC.
Formula Explanation: A brief description of how EFW is determined.
Growth Chart: A visual representation of your baby's estimated weight against typical growth curves.
Growth Table: Standard percentile ranges for comparison.
How to read results: Compare your baby's estimated weight and the estimated age from AC against standard growth charts and percentile tables. Your healthcare provider will interpret these results in the context of your overall pregnancy health.
Decision-making guidance: An EFW estimate is a diagnostic aid, not a definitive prediction. Significant deviations from expected growth, or inconsistencies between measurements, should always be discussed with your doctor or midwife. They will use this information, along with other clinical factors, to make informed decisions about your care.
Key Factors That Affect Estimated Foetal Weight Results
While ultrasound measurements and formulas are the basis for EFW, several factors influence the accuracy and interpretation of the results:
Accuracy of Ultrasound Measurements: This is paramount. Fetal positioning, maternal body habitus (e.g., obesity), operator experience, and the quality of the ultrasound equipment can all affect the precision of the BPD, HC, AC, and FL measurements. Even small errors can lead to noticeable variations in the EFW.
Gestational Age Accuracy: The initial dating of the pregnancy, usually based on the first-trimester ultrasound or Last Menstrual Period (LMP), sets the benchmark. If the dating is off, the interpretation of the EFW relative to "normal" for that age will be skewed. The AC is often used to cross-check gestational age.
Fetal Anatomy Variations: Some babies naturally have different body proportions. For example, a baby with a smaller head but larger abdomen might have an AC that suggests a younger gestational age than their BPD or FL. Formulas try to account for this, but extreme variations can challenge them.
Fetal Growth Patterns (SGA/LGA): Fetuses experiencing Intrauterine Growth Restriction (IUGR) or those identified as Large for Gestational Age (LGA) may not follow the standard growth curves used in the EFW formulas. The formulas are derived from average populations, and outliers can be less accurately estimated.
Maternal Factors: Maternal health conditions like diabetes (can lead to macrosomia) or malnutrition/smoking (can lead to IUGR) significantly impact fetal growth. While EFW can reflect these impacts, the formulas themselves don't adjust for maternal conditions directly.
Placental Function: The placenta is the lifeline for the fetus. If placental function is compromised, nutrient and oxygen delivery may be reduced, leading to slowed growth, which EFW measurements can detect. Conversely, a hyper-functioning placenta might contribute to accelerated growth.
Oligohydramnios/Polyhydramnios: Low (oligohydramnios) or excessive (polyhydramnios) amniotic fluid can sometimes make obtaining accurate measurements more challenging, potentially affecting EFW accuracy.
Formulas Used: As mentioned, different EFW formulas (Hadlock, Shepard, Merks, etc.) exist, each based on different datasets and mathematical models. They can produce slightly different results. The choice of formula often depends on the gestational age range and local clinical practice.
Frequently Asked Questions (FAQ)
What is the most accurate EFW formula?
There isn't one single "most accurate" formula for all situations. The Hadlock formula is widely used and considered reliable, particularly in the third trimester. However, the best formula can depend on the specific gestational age and the particular measurements obtained. Healthcare providers often use software that incorporates multiple formulas to provide a range.
How much error is acceptable in EFW?
A typical margin of error for EFW estimations is around +/- 10% to 15%. This means a baby estimated at 2000g could realistically be anywhere between 1700g and 2300g. Factors like operator skill and fetal position play a significant role.
Can EFW predict delivery complications?
Extremely high or low EFW estimates can indicate potential complications. Macrosomia (very large baby) might suggest a higher risk of C-section or birth injuries. Severe IUGR can indicate fetal distress and may necessitate early delivery. However, EFW is just one factor considered.
Why is the Abdominal Circumference (AC) so important?
The AC is strongly correlated with fetal liver glycogen and adipose tissue deposition, which are major determinants of fetal weight, especially in the later stages of pregnancy. It is often considered the most critical single measurement for EFW estimation and is also used to assess fetal age.
My baby's AC suggests an earlier gestational age than my due date. What does this mean?
This discrepancy can suggest that the baby's growth is not keeping pace with the expected rate for the established gestational age. It might indicate fetal growth restriction (FGR) or that the initial dating of the pregnancy was inaccurate. Your healthcare provider will investigate further.
How often should EFW be monitored?
Routine EFW monitoring isn't standard for all pregnancies. It's typically performed if there are concerns about fetal growth (e.g., fundal height measurement doesn't match dates, history of IUGR) or maternal conditions like diabetes. Monitoring frequency depends on the clinical situation, often every 2-4 weeks if indicated.
Can I use this calculator at home without an ultrasound?
No. This calculator requires specific biometric measurements (BPD, HC, AC, FL) that can ONLY be obtained accurately through an ultrasound scan performed by a trained professional.
Does EFW tell me the baby's gender?
No, EFW calculations are based purely on physical measurements and have no relation to the baby's sex. Gender determination requires different ultrasound techniques.
Related Tools and Internal Resources
Due Date Calculator – Calculate your estimated due date based on your last menstrual period.
Pregnancy Trimester Guide – Learn about the developmental milestones during each stage of pregnancy.