Hadlock Estimated Fetal Weight Calculator

Accurately estimate fetal weight using ultrasound measurements with the Hadlock formula.

Fetal Weight Calculator

EFW vs. Gestational Age (Illustrative Comparison)

Key Input Measurements

Measurement	Value (cm)
Biparietal Diameter (BPD)	—
Occipitofrontal Diameter (OFD)	—
Abdominal Circumference (AC)	—
Femur Length (FL)	—

What is the Hadlock Estimated Fetal Weight Calculator?

The Hadlock Estimated Fetal Weight (EFW) calculator is a vital tool used in prenatal care to estimate the weight of a fetus during pregnancy. It's based on the widely recognized Hadlock formula, developed by Dr. Robert L. Hadlock and colleagues, which utilizes measurements obtained from an ultrasound scan. This calculator provides healthcare professionals, and expectant parents, with an approximation of the baby's size, aiding in monitoring fetal growth and identifying potential growth abnormalities. It's crucial to understand that this is an *estimate*; actual birth weight can vary. The calculator typically takes four key fetal measurements: the biparietal diameter (BPD), occipitofrontal diameter (OFD), abdominal circumference (AC), and femur length (FL).

Who Should Use It?

The primary users of the Hadlock Estimated Fetal Weight calculator are obstetricians, maternal-fetal medicine specialists, sonographers, and other healthcare providers involved in prenatal diagnosis and monitoring. Expectant parents may also find it useful for understanding their baby's growth trajectory as discussed with their doctor. It's particularly helpful for:

• Monitoring fetal growth throughout pregnancy.
• Detecting suspected intrauterine growth restriction (IUGR) or macrosomia (large for gestational age).
• Guiding clinical decisions, such as timing of delivery or interventions.
• Assessing fetal well-being in high-risk pregnancies.

Common Misconceptions

A common misconception is that the EFW calculator provides an exact birth weight. It's important to remember it's an estimate with a margin of error that can increase as the pregnancy progresses. Another misconception is that it's a standalone diagnostic tool; it's always used in conjunction with other clinical information, fetal monitoring, and the healthcare provider's expertise. It's also not a substitute for regular prenatal check-ups.

Hadlock Estimated Fetal Weight Calculator Formula and Mathematical Explanation

The Hadlock formula is a regression equation derived from a large dataset of ultrasound measurements. It's designed to predict fetal weight based on specific biometric parameters measured during an ultrasound. The most commonly cited version of the Hadlock formula for EFW in grams is:

EFW (grams) = 10^{(1.3596 + 0.02276 * AC + 0.0188 * BPD + 0.00418 * FL)}

This formula calculates the estimated fetal weight directly. Often, obstetricians will also calculate the standard deviation (SD) and fetal weight percentile to compare the estimated weight against expected norms for the gestational age. While the direct EFW calculation is straightforward with the above formula, determining the SD and percentile typically requires using a nomogram (a graphical chart) or statistical software calibrated to the Hadlock dataset.

Variable Explanations

Variables in the Hadlock EFW Formula

Variable	Meaning	Unit	Typical Range (Approximate)
EFW	Estimated Fetal Weight	Grams (g)	Varies greatly by gestational age
AC	Abdominal Circumference	Centimeters (cm)	~5 cm (20 weeks) to ~35+ cm (40 weeks)
BPD	Biparietal Diameter	Centimeters (cm)	~5 cm (20 weeks) to ~9+ cm (40 weeks)
FL	Femur Length	Centimeters (cm)	~3.5 cm (20 weeks) to ~8+ cm (40 weeks)
OFD	Occipitofrontal Diameter	Centimeters (cm)	~6.5 cm (20 weeks) to ~11+ cm (40 weeks)

Note: The typical ranges are approximate and will vary significantly based on the stage of pregnancy. The OFD was sometimes used in earlier versions or variations of fetal weight formulas; the primary Hadlock formula used here relies on AC, BPD, and FL. This calculator uses BPD, AC, and FL for EFW, and includes OFD as an input for potential future formula expansions or variations. The provided calculator focuses on the core EFW calculation using AC, BPD, and FL.

Practical Examples (Real-World Use Cases)

Example 1: Monitoring Normal Growth

A pregnant patient at 30 weeks of gestation undergoes an ultrasound. The measurements are:

• BPD: 7.8 cm
• AC: 27.5 cm
• FL: 5.8 cm

Using the Hadlock calculator:

EFW = 10^{(1.3596 + 0.02276 * 27.5 + 0.0188 * 7.8 + 0.00418 * 5.8)}

EFW = 10^{(1.3596 + 0.626 + 0.1466 + 0.0242)}

EFW = 10^(2.1504) ≈ 141.4 cm

This yields an EFW of approximately 1414 grams (or 1.41 kg). This weight would be compared to standard growth charts for 30 weeks to determine if it falls within the normal percentile range. If it does, it suggests normal fetal growth.

Example 2: Suspected Macrosomia (Large Baby)

Another patient at 38 weeks of gestation has ultrasound measurements suggesting a larger-than-average baby:

• BPD: 9.5 cm
• AC: 34.0 cm
• FL: 7.5 cm

Using the Hadlock calculator:

EFW = 10^{(1.3596 + 0.02276 * 34.0 + 0.0188 * 9.5 + 0.00418 * 7.5)}

EFW = 10^{(1.3596 + 0.7738 + 0.1786 + 0.0314)}

EFW = 10^(2.3434) ≈ 220.5 cm

This results in an EFW of approximately 2205 grams (or 2.2 kg). This value would be assessed against the typical weight for 38 weeks. If it's significantly above the 90th percentile, it might lead to a diagnosis of macrosomia, prompting discussions about delivery methods, potential complications, and maternal health management.

How to Use This Hadlock Estimated Fetal Weight Calculator

Using our Hadlock EFW calculator is straightforward:

1. Gather Ultrasound Measurements: Obtain the accurate measurements of the fetus's BPD, AC, and FL (in centimeters) from your most recent ultrasound report. You may also have an OFD measurement, though it's not directly used in the primary Hadlock EFW formula implemented here.
2. Input Data: Enter each measurement into the corresponding field in the calculator. Ensure you enter the correct values and units (cm).
3. Calculate: Click the "Calculate Weight" button.
4. View Results: The calculator will display the Estimated Fetal Weight (EFW) in grams. It will also show approximate Standard Deviation (SD), Estimated Gestational Age (EGA), and Fetal Weight Percentile if these can be derived from the inputs and associated nomograms (note: this specific calculator primarily focuses on EFW from inputs). The results will also populate a table below.
5. Interpret Results: Discuss the calculated EFW with your healthcare provider. They will compare it to expected fetal growth charts for your specific gestational age to assess your baby's growth pattern.
6. Reset or Copy: Use the "Reset" button to clear the fields and start over. Use the "Copy Results" button to save or share the calculated information.

How to Read Results: The main result is the EFW in grams. The SD and Percentile help contextualize this weight. For example, a baby at the 50th percentile weighs approximately the average for their gestational age, while a baby at the 90th percentile is larger than 90% of fetuses of the same age. Always rely on your doctor's interpretation.

Decision-Making Guidance: The EFW is one piece of information. Significant deviations from the expected growth curve may prompt further investigation or discussion about birth plans. For instance, suspected macrosomia might influence decisions about vaginal delivery versus Cesarean section, while suspected IUGR might lead to increased fetal monitoring.

Key Factors That Affect Hadlock Estimated Fetal Weight Results

While the Hadlock formula is robust, several factors can influence the accuracy of the EFW estimate and the actual fetal growth:

1. Gestational Age Accuracy: The accuracy of the gestational age derived from the Last Menstrual Period (LMP) or early ultrasound is fundamental. Errors here can skew the interpretation of EFW.
2. Sonographer Skill and Equipment: The precision of the ultrasound measurements (BPD, AC, FL) is critical. Operator experience, probe quality, and patient factors (like maternal body habitus) can affect measurement accuracy.
3. Fetal Position: The way the baby is positioned in the uterus can sometimes make certain measurements harder to obtain accurately.
4. Maternal Health Conditions: Conditions like gestational diabetes can lead to increased fetal size (macrosomia), while others like hypertension or placental insufficiency can impair growth (IUGR).
5. Placental Function: A healthy placenta is crucial for delivering nutrients and oxygen to the fetus. Impaired placental function can significantly slow fetal growth.
6. Maternal Nutrition: Adequate and balanced nutrition is essential for fetal development. Poor maternal nutrition can contribute to restricted growth.
7. Genetic Factors: Both parents' genetic makeup can influence the baby's potential size.
8. Uterine Factors: Uterine anomalies or leiomyomas (fibroids) can sometimes affect fetal growth space and development.

These factors highlight why EFW is an estimate and why clinical context is vital for accurate interpretation.

Frequently Asked Questions (FAQ)

Q1: How accurate is the Hadlock estimated fetal weight calculator?

A1: The Hadlock formula is considered one of the most accurate methods for EFW estimation. However, it's an estimate, and the typical margin of error is around ±10-15%. Accuracy can decrease with advanced gestational age and in cases of fetal growth abnormalities.

Q2: Can I use the Hadlock calculator at home?

A2: Yes, you can use this calculator if you have the measurements from your ultrasound. However, always discuss the results with your healthcare provider, as they can interpret them within the full context of your pregnancy.

Q3: What if my baby's EFW is very high or very low?

A3: A significantly high EFW might suggest macrosomia (large baby), while a low EFW could indicate intrauterine growth restriction (IUGR). Your doctor will likely recommend further monitoring or tests to understand the cause and manage the situation appropriately.

Q4: Does the calculator predict the exact birth weight?

A4: No, it provides an estimate. The actual birth weight can differ due to various factors, including the baby's positioning, fluid levels, and other physiological variables not captured by the formula.

Q5: Why are BPD, AC, and FL used in the formula?

A5: These measurements are chosen because they correlate well with fetal weight across different stages of pregnancy. BPD and FL give an indication of skeletal size, while AC reflects soft tissue and abdominal development, which is crucial for overall weight gain.

Q6: Are there other fetal weight formulas?

A6: Yes, there are other formulas (e.g., Shepard, Warsof), but Hadlock's is widely adopted due to its accuracy and validation across diverse populations. Different institutions may use variations or specific nomograms.

Q7: When is an EFW typically performed?

A7: EFW assessments are often done as part of routine second and third-trimester ultrasounds (around 20 weeks and onward) or more frequently if there are concerns about fetal growth or maternal conditions.

Q8: What is the difference between EFW and percentile?

A8: EFW is the calculated weight in grams. Percentile indicates how the EFW compares to other fetuses of the same gestational age. For example, the 75th percentile means the fetus weighs more than 75% of fetuses at that age.

Related Tools and Internal Resources