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How do you calculate fetal weight? Try using an estimated fetal weight calculator

Use an estimated fetal weight calculator and keep an eye on baby throughout your pregnancy

How much your baby weighs while in the womb is an important marker not only for fetal development, but for wellness outcomes during childhood and adolescence. Low birth weight affects the brain development 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. It’s a pretty big deal.

Monitoring fetal weight during pregnancy is important for more reasons than just helping to determine how the actual birth is going to go. Low birth weight is related to undesirable outcomes in the delivery room. Adjusting the mother’s care plan to hopefully increase fetal weight before delivery is in everyone’s best interests. We’ll help you navigate determining your baby’s weight in the womb and why that matters.

A pregnant woman using a monitor on her belly at home.

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 until that baby comes out. But doctors have established algorithms based on ultrasound measurements and clinical observations that gives 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 is close to plus or minus ten percent. But even an educated guess is best when wanting to keep mother and baby as healthy as possible.

pregnant -woman-timing-contractions

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 medical degree. Or any degree at all. There are fetal weight calculators online where you plug in a few numbers and the program does all the heavy lifting. You do need to know some basic information about your pregnancy and your baby’s size.

Even though the final numbers won’t be completely accurate, if you have concerns about your baby’s weight or development, discuss them with your health care provider to determine if you should make changes in your care plan.

A pregnant woman using a monitor on her stomach.

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 of your peanut. Ask your ultrasound technician to write them down for you at your next appointment, and every appointment after, so you can keep an up-to-date account of your baby’s growth.

Measurements to determine fetal weight:

  • Gestational age: the number of weeks and days pregnant you are
  • Biparietal diameter: the diameter of the head side to side
  • Head circumference: the distance around the entire head
  • Abdominal circumference: the distance from the top of the pubic bone to the top of the belly
  • Femur length: the length of the femur, or thigh bone
  • Race: after the 20th week of pregnancy, fetal weight differs greatly by race

These measurements are generally given in millimeters, but if yours aren’t, you’ll need to convert them. Once you have your baby’s measurements, you just pop them into the calculator.

The calculator will provide you with a range of percentiles for each category and fetal weight estimates based on four different calculations. Each weight is also accompanied by a percentile. These numbers all carry the almost 10% margin of error previously mentioned. Fetal weight calculators are guidelines, and the weights provided are not set in stone. If your results worry you, discuss them with your healthcare provider.

A pregnant woman talking to her health care 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, getting adequate sleep, and exercising will improve the health of your nugget during pregnancy. If your fetal weight is too low, your doctor will discuss steps to take to increase weight gain prior to birth.

Above all, it’s important to communicate with your health care team about any concerns you may have. But don’t let stress about your baby’s birth weight negatively impact your mental health. The chance of your baby being bigger or smaller than the predicted birth weight is normal. Just like sometimes the sex is incorrectly determined or there are surprise twins, your baby can come out weighing more or less than predicted.

Also, percentiles are just that, percentiles. If your baby is measuring in the 60th percentile, that means they are measuring bigger than 60% of babies at the same gestational age, with the 50th percentile being average. 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.

If you want to know additional aspects of your child’s development, want to share some extra information with your partner, or need to keep closer track of your precious bundle for health reasons, knowing how to use a fetal weight calculator is a good idea. Keep yourself and your pregnancy on track and have fun seeing if your nugget comes out close to the weight you’ve calculated.

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Kristi Pahr
Former Digital Trends Contributor
Kristi is a professional writer and mother of two. When she's not writing or playing chauffeur to her kids, she enjoys…
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