How do you calculate fetal weight? Try using this simple calculator

Fetal weight is an important marker not only for fetal development but for wellness outcomes during childhood and adolescence. Low birth weight can affect the brain development of the baby in infancy and childhood. Studies have shown that low birth weight has an impact on cerebral cortex development well into adolescence. The cerebral cortex is the area of the brain that is responsible for functions such as consciousness, thought, emotion, reasoning, language, and memory.

Monitoring fetal weight during pregnancy is important for more reasons than just determining the route of delivery when the time comes. Knowing that low birth weight is related to undesirable outcomes, both practitioners and parents can adjust the mother’s care plan to hopefully increase fetal weight before delivery.

How do health care providers determine fetal weight

Fetal weight calculations are only estimates. There is no way to be 100% accurate in determining fetal weight, but doctors have established algorithms based on ultrasound measurements or clinical observations that can give them a ballpark when it comes to determining fetal weight.

There are several formulas for determining fetal weight and your health care provider will use the one they are most comfortable with. The margin of error for fetal weight estimates is close to plus or minus 10% so it really is more of an estimate than a hard and fast number.

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Getty Images / JGI/Jamie Grill

Determining fetal weight at home

If you want to try your hand at determining your baby’s fetal weight at home, you don’t have to have an advanced math degree or medical degree. There are fetal weight calculators online where you plug in a few numbers and the program does all the heavy lifting.

It’s important to remember that the results are only estimates. If you have concerns about your baby’s weight or development, you should discuss them with your health care provider and determine if you should make changes in your care plan.

How to use a fetal weight calculator

The National Institute of Child Health and Human Development’s fetal growth calculator is a straightforward and easy-to-use program to determine your baby’s estimated fetal weight. Based on the results, the calculator will provide you with your baby’s weight percentile.

To use the calculator, you’ll need a few measurements. Ask your ultrasound technician to write them down for you at your next ultrasound appointment.

Measurements to determine fetal weight:

  • Gestational age: the number of weeks pregnant you are
  • Biparietal diameter: the diameter of the head side to side
  • Head circumference: the distance around the entire head
  • Femur length: the length of the femur, or thigh bone

You will also need to enter your race.

These measurements are generally given in millimeters but if yours aren’t, you’ll need to convert them so you get an accurate fetal weight estimate. Once you have your baby’s measurements, you just plug them into the calculator.

The calculator will then provide you with estimates on percentiles for each estimate and fetal weight estimates based on four different calculations. Each weight is also accompanied by a percentile. It’s important to remember that these numbers are estimates only and carry the almost 10% margin of error previously mentioned. Fetal weight calculators are guidelines only, and the weights provided are not set in stone. If your results worry you, discuss them with your healthcare provider.

How to increase fetal weight

The most important thing you can to do bolster your baby’s health is to be healthy yourself. Eating a balanced diet, staying hydrated, taking prenatal vitamins, and getting adequate sleep and exercise can improve the health of your pregnancy. If your fetal weight is very low, your doctor will discuss steps to take to increase weight gain prior to birth.

It’s worth repeating though, that fetal weights are only estimates and the chances of your baby being bigger or smaller than the predicted birth weight are reasonable. Also, percentiles are just that, percentiles. If your baby is measuring in the 60th percentile, that means they are measuring bigger than 60% of other babies at the same gestational age. The 50th percentile is average and higher percentiles don’t necessarily mean better percentiles. A baby in the 99th percentile is bigger than almost every other baby of the same gestational age and that could mean changes in delivery plans or a switch to a surgical birth instead of a vaginal one.

Above all, it’s important to communicate with your health care team about any concerns you may have and don’t let stress about your baby’s birth weight negatively impact your mental health.

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