Fórmula Johnson Para Calcular Peso Fetal

The Johnson formula is a widely used method in obstetrics to estimate fetal weight based on ultrasound measurements. It provides a quick and reliable estimate that can be useful in prenatal care and monitoring fetal growth.

What is the Johnson formula?

The Johnson formula is an empirical equation developed to estimate fetal weight from biparietal diameter (BPD) and abdominal circumference (AC) measurements obtained via ultrasound. It was first published in 1973 and remains one of the most commonly used methods in obstetrics.

Johnson Formula

Fetal weight (grams) = 1,340 + 9.31 × (BPD in cm) + 7.28 × (AC in cm) - 6.71 × (BPD in cm)²

The formula combines these two key measurements to provide an estimate of fetal weight. BPD is measured as the distance between the two parietal bones of the fetal skull, while AC represents the circumference around the fetus's abdomen.

How to use the Johnson formula

To use the Johnson formula, you'll need:

Biparietal diameter (BPD) measurement in centimeters
Abdominal circumference (AC) measurement in centimeters

Follow these steps:

Measure the BPD and AC using ultrasound equipment
Convert the measurements to centimeters if they're in millimeters
Plug the values into the formula
Calculate the result

For best results, use measurements obtained between 18 and 22 weeks of gestation. The formula is most accurate during this period when fetal growth is relatively predictable.

Example calculation

Let's say you have a fetus with:

Biparietal diameter (BPD) = 4.2 cm
Abdominal circumference (AC) = 28.5 cm

Plugging these values into the formula:

Fetal weight = 1,340 + (9.31 × 4.2) + (7.28 × 28.5) - (6.71 × 4.2²)

Calculating each term:

9.31 × 4.2 = 39.082
7.28 × 28.5 = 207.16
4.2² = 17.64
6.71 × 17.64 = 118.1624

Now combine all terms:

1,340 + 39.082 + 207.16 - 118.1624 = 1,468.08 grams

Convert grams to kilograms: 1,468.08 ÷ 1,000 = 1.468 kg

Example Result

For a fetus with BPD of 4.2 cm and AC of 28.5 cm, the estimated fetal weight is approximately:

1.47 kg

Limitations of the formula

While the Johnson formula is widely used, it has some limitations:

It's most accurate between 18-22 weeks of gestation
Accuracy decreases outside this gestational range
It doesn't account for fetal sex or multiple pregnancies
Results may vary between different ultrasound machines
It provides an estimate, not an exact measurement

For more precise fetal weight estimation, consider using alternative methods like Hadlock formula or Hadlock II formula, which may provide more accurate results in certain situations.

FAQ

What is the Johnson formula used for?

The Johnson formula is primarily used to estimate fetal weight based on ultrasound measurements of biparietal diameter and abdominal circumference. It's commonly used in prenatal care to monitor fetal growth and development.

Is the Johnson formula accurate?

The Johnson formula provides a reasonable estimate of fetal weight, especially between 18-22 weeks of gestation. However, its accuracy can vary depending on the individual fetus and specific ultrasound measurements.

What measurements are needed for the Johnson formula?

You need two key measurements: biparietal diameter (BPD) and abdominal circumference (AC), both typically obtained via ultrasound.

Can the Johnson formula be used for all pregnancies?

The formula is most accurate for singleton pregnancies between 18-22 weeks of gestation. It may not be as reliable for multiple pregnancies or outside this gestational window.

What if the Johnson formula gives an unrealistic result?

If the result seems unrealistic, double-check your measurements and ensure they're within the normal range for the given gestational age. Consider using alternative formulas or consulting with a healthcare professional.