Estimated Fetal Weight Calculator (Using Fundal Height)

An easy tool to calculate estimated fetal weight based on Symphysis-Fundal Height (SFH) measurements. This calculator helps you understand how to calculate estimated fetal weight using fundal height.

What is Estimating Fetal Weight with Fundal Height?

Estimating fetal weight using fundal height is a clinical method used during pregnancy to gauge a baby’s growth. Fundal height, also known as Symphysis-Fundal Height (SFH), is the measurement in centimeters from the top of the pubic bone (symphysis pubis) to the top of the uterus (the fundus). This measurement helps healthcare providers like midwives and obstetricians track fetal development, especially after 20-24 weeks of gestation.

It’s important to understand that this is an estimation, not a diagnostic tool. While it’s a simple, non-invasive, and useful screening method, it is less precise than an ultrasound. Common misunderstandings arise when the fundal height doesn’t exactly match the gestational week. Many factors can influence this measurement, so it is used as one of several data points to monitor a healthy pregnancy.

The Formula for Estimated Fetal Weight (Johnson’s Rule)

One of the most common clinical formulas used to calculate estimated fetal weight from fundal height is Johnson’s Rule. This formula is simple and relies on the fundal height measurement and whether the baby’s head has “engaged” or dropped into the mother’s pelvis.

The formula is: EFW (grams) = (SFH in cm – n) * 155

Where ‘n’ is a correction factor based on engagement. This calculator uses this widely recognized formula to provide an estimate.

Variable Explanations for Johnson’s Rule

Variable	Meaning	Unit	Typical Range
EFW	Estimated Fetal Weight	grams (g)	500 – 4500 g
SFH	Symphysis-Fundal Height	centimeters (cm)	20 – 42 cm
n	Correction Factor	Unitless	11 (Not Engaged) or 12 (Engaged)

Practical Examples

Example 1: Mid-Third Trimester, Head Not Engaged

• Inputs: Gestational Age: 34 weeks, Fundal Height: 33 cm, Head Engaged: No
• Calculation: n = 11. EFW = (33 – 11) * 155 = 22 * 155 = 3410 grams.
• Result: The estimated fetal weight is approximately 3410 grams, or about 7 lbs 8 oz.

Example 2: Near Term, Head Engaged

• Inputs: Gestational Age: 39 weeks, Fundal Height: 37 cm, Head Engaged: Yes
• Calculation: n = 12. EFW = (37 – 12) * 155 = 25 * 155 = 3875 grams.
• Result: The estimated fetal weight is approximately 3875 grams, or about 8 lbs 9 oz.

How to Use This Estimated Fetal Weight Calculator

Our calculator simplifies the process to calculate estimated fetal weight using fundal height. Follow these steps for an accurate estimation:

1. Enter Gestational Age: Input the number of weeks you are into your pregnancy. This is used for plotting the result against average growth charts.
2. Enter Fundal Height: Input the Symphysis-Fundal Height (SFH) measurement in centimeters. This should be measured by a healthcare provider for best accuracy.
3. Select Head Engagement Status: Choose “Yes” or “No” from the dropdown. This adjusts the formula’s correction factor for a more refined estimate.
4. Interpret the Results: The calculator will instantly display the estimated fetal weight in both grams and pounds/ounces. It also shows the intermediate values used in the calculation for full transparency. The result is also shown on a chart for comparison with average fetal weights.

Key Factors That Affect Fundal Height Accuracy

Several factors can influence the accuracy of a fundal height measurement, which is why it’s considered a screening tool rather than a diagnostic one. A measurement that is larger or smaller than expected doesn’t automatically mean there is a problem.

• Maternal BMI: A higher Body Mass Index can sometimes lead to a less accurate measurement.
• Amniotic Fluid Level: Too much (polyhydramnios) or too little (oligohydramnios) amniotic fluid will alter the size of the uterus.
• Fetal Position (Lie): A transverse (sideways) or breech (feet-first) position can affect the uterine shape and measurement.
• Multiple Gestation: Carrying twins or more will naturally result in a larger fundal height.
• Full Bladder: A full bladder can push the uterus upwards, artificially increasing the measurement. It’s best to measure with an empty bladder.
• Measurement Technique: Consistency is key. Different people may measure slightly differently, leading to small variations.

For more information, you might find our Pregnancy Due Date Calculator useful.

Frequently Asked Questions (FAQ)

How accurate is using fundal height to estimate fetal weight?

It’s an estimation. Studies show it’s reasonably accurate for predicting a weight range but can have a significant margin of error compared to ultrasound. It’s most useful for tracking a growth trend over time.

Why is my fundal height measurement different from my gestational age?

After about 24 weeks, fundal height in cm often corresponds to the week of gestation, plus or minus 2-3 cm. A discrepancy could be due to the factors listed above, or it could indicate the baby is larger or smaller than average, which might prompt an ultrasound for a closer look.

What does ‘head engaged’ mean?

This means the widest part of the baby’s head has descended into the pelvic brim in preparation for birth. It often happens in the last few weeks of pregnancy, especially for first-time mothers. This is also known as “lightening.”

Can this calculator predict my baby’s actual birth weight?

No, it provides an estimate at a specific point in time. A baby’s growth can speed up or slow down in the final weeks. This calculator to calculate estimated fetal weight using fundal height is a guide, not a guarantee.

Is this a replacement for an ultrasound?

Absolutely not. An ultrasound, which uses measurements like head circumference (HC), abdominal circumference (AC), and femur length (FL), is a much more accurate method for estimating fetal weight. Fundal height is a low-tech screening tool.

What if my baby is measuring large or small?

Your healthcare provider will interpret the measurement in the context of your entire pregnancy. If there’s a concern, they may recommend an ultrasound to get a more precise measurement and assess the baby’s well-being. Our Fetal Growth Chart guide provides more context.

What is a normal fundal height?

From about week 24, a normal fundal height is typically within 2-3 centimeters of your gestational week. For example, at 30 weeks, a measurement between 27 and 33 cm is usually considered normal. Explore more about this topic in our article on Understanding Fetal Percentiles.

At what week do you start measuring fundal height?

Providers typically begin measuring fundal height at prenatal appointments starting around week 20 to 24 of pregnancy.