Fetal weight: how it is calculated and why it is an important parameter

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Catherine Le Nevez
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Day after day the fetus grows inside the mother's belly and prepares for the extra-uterine life. "The fetal weight it is a parameter that can be calculated at each ultrasound scan and which is compared with specific tables to evaluate the correct growth of the child »explains gynecologist Marinella Dell'Avanzo, expert in obstetric ultrasound, of Humanitas San Pio X.



In this article

 



  • The calculation of the fetal weight, 
  • The growth charts,
  • How much and how does the fetal weight increase.

The calculation of the fetal weight

«Weight is not an element that is taken into consideration in early pregnancy. Until the 12th week of gestation, we only measure the length of the fetus, considering the distance from the head to the butt of the baby (CRL). (Go to our online fetus length calculator).



Read also: The development of the fetus in the 40 weeks

Subsequently, you manage to get one pixy Of weight automatically thanks to the sophisticated ultrasound machines in use today. The data is processed on the basis of specific formulas. «The most used is the formula in Hadlock, which calculates the fetal weight on the basis of a combination of parameters: the biparietal diameter (or BPD), the head circumference (CC); the abdominal circumference (CA); and the length of the femur (FL). Thanks to these four elements the computer, inserted in the ultrasound machines, indicates which is the presumed fetal weight».



Among those indicated, the most important parameter is the abdominal circumference: «It is a good idea to make at least three measurements of the abdomen in order to obtain a mathematical average that the machine uses to calculate the presumed fetal weight».

Different methods


The calculation of Hadlock, invented in the 70s, is not the only one used for fetal weight assessment. The formula of Campbell e Wilkin evaluates only the abdominal circumference, while the Shephard's formula takes into account the biparietal diameter (BPD) and the abdominal circumference (CA).

The growth charts

The next step is to evaluate the data obtained by placing them on growth curves or tables.

Biparietal diameter (BPD), head circumference (CC), abdominal circumference (CA), femur (FL) and fetal weight are placed on the percentiles correspondents to see if the fetus is growing correctly.

These tables, identified at a general level by the World Health Organization (but there are also local ones), are fundamental indicators for obstetricians and differ according to various factors such as the sex of the fetus, ethnicity and origin. geographical area of ​​the parents, parity (i.e. the number of previous children). Some also take into account the anthropometric characteristics (weight, height, constitution) of the parents.

Read also: Fetal growth retardation: what it is, how to intervene, what are the prospects for the future

Each clinical decision on the data obtained from the ultrasound is to be done with the gynecologist specialist. «Don't be frightened if the data reported on the growth curves are not in the range. The results will be shared with the gynecologist, who will decide on the next clinical course (for example, he will ask for further checks). Sometimes it is necessary to repeat the ultrasound after a few weeks ".

The weight is assumed

It must be remembered that the fetal weight identified by the ultrasound is always alleged, «This means that it is never 100% correct, but that it has a margin of error of 10% more or less. The child, therefore, could weigh up to 10% more or less than the data obtained from the ultrasound ".

How much and how does the fetal weight increase

The weight of the fetus tends to increase by approximately 100 grams per week up to the 30th week; after that it grows even faster.

Between the 19th and 21st weeks of pregnancy, whenmorphological ultrasound, the fetus weighs about 250-350 grams for a total length of about 20 centimeters.

Sometimes it is necessary to do an ultrasound a 34 and 36 weeks of pregnancy to evaluate the approximate weight of the child and his abdominal circumference (AC) to avoid the risks associated with fetal macrosomia, that is, the problems associated with giving birth with a fetus that is too large.


If it is estimated that the weight of the fetus is over 4,2 kg, the gynecologist may consider inducing delivery and, in some cases, may also decide for a caesarean section.

Read also: Fetal macrosomia, what to do if the fetus is large

Questions and answers

When is the fetal weight calculated? 

Until the 12th week of gestation, we only measure the length of the fetus, considering the distance from the head to the butt of the baby (CRL). Subsequently, it is possible to obtain an estimate of the weight automatically thanks to the sophisticated ultrasound machines in use today.

Calculation of fetal weight: what is Hadlock's formula? 

Hadlock's formula calculates the fetal weight on the basis of a combination of parameters: the biparietal diameter (or BPD), the head circumference (CC); the abdominal circumference (CA); and the length of the femur (FL). 

How and how much does the fetal weight increase?

The weight of the fetus increases by about 100 grams per week until the 30th week; after that it grows even faster. 

TAG:
  • fetus weight calculation
  • development of the fetus
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