How to Calculate Birth Weight

The baby’s birth weight is an important factor to determine several possible problems the baby may have. A baby can be considered of low birth weight (LBW) when their weight is below 5.51 lbs or 2500 grams at birth. If the baby weighs 2.20 lbs- 3.30 lbs (1000-1500 grams) they are classified as very low birth weight (VLBW). When the baby weighs below 1000 grams (2.20 lbs) then they are classified as extremely low birth weight (ELBW). These LBW, VLBW and ELBW babies have high risks of developing respiratory, exercise, nutritional and functional problems. To be able to determine the baby’s birth weight during the third trimester of your pregnancy can help reduce the risks of having a low birth weight after giving birth. Determining the baby’s birth weight also helps to estimate if you would need a c-section, induction or episiotomy.



Step 1. Schedule an ultrasound test when you reach 28-42 weeks to determine if you have a single or multiple gestations and other data like the abdominal circumference. Look for a clinic that offers professional ultrasound services. You might want to consider these professional Ultrasound services in Bloomsburg.

Step 2. Buy online the Birthweight Estimate Calculator (see Resources for the website) to determine the estimated weight of your baby. You will have to enter the data that will be asked of you such as your last monthly period (LMP), maternal weight at 26 and 30 weeks and current maternal weight. However, the calculator can only give precise estimate for a singleton or single gestation.

Birth Weight

Step 3. Download the software that is available at Gestation Network (see Resources for the website) which needs data from the ultrasound test like the abdominal circumference, head circumference and femur length to compute the estimated birth weight.

Step 4. Enter the data that will be asked in the software like the fetal biometric measurements that can be found in your ultrasound report. The results will be displayed after you enter the data.

Step 5. To calculate the birth weight using the Gestation- Adjusted Projection (GAP), which has been identified by physicians at the University of Rochester Medical Center, uses data from the ultrasound images taken in the 34th and 36th week of pregnancy. Your doctor will then use the data in a mathematical formula to estimate the birth weight of your baby.


  • You must take time to check you baby’s birth weight for the early detection of the possibility of having a low birth weight. This can be intervened and eventually avoid birth complications.
  • During your pregnancy, you must avoid becoming overweight since this can affect the weight of your baby. And it can also lead to complications when you will give birth.
  • Make sure that the information you will enter to calculate the birth weight is accurate so that the result will also be precise. You should take note that there are several factors that might affect the results such as women with pregnancy- related illnesses, race and if you are a smoker.
  • You must have a regular check up with your doctor during your pregnancy, especially during the third trimester. Your doctor can also make a rough estimate of your baby’s birth weight based on the weight you have gained from the previous check up.


  • Strenuous exercises during pregnancy can lead to premature births and low birth weight.
  • The Birthweight Estimate Calculator is only an estimate, it is still best to see your doctor for consultation.
  • Babies with low birth weight have high risks of having a respiratory problem such as bronchiolitis which have asthma- like symptoms.

Filed Under: Health & Personal Care


About the Author: Andrew Reinert is a health care professional who loves to share different tips on health and personal care. He is a regular contributor to MegaHowTo and lives in Canada.

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