Biparietal fetus diameter (BPD), what it is

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The fetus grows more and more in the mother's womb every day. For this reason it is essential to constantly monitor it - detecting through specific tests - the lunghezza (up to the 12th week of gestation) and, subsequently, the weight. In order to identify the correct development of the fetus and, above all, the right weight of the baby, it is necessary to detect several important parameters, the combination of which offers the final value: among these is the biparietal diameter (or BPD). 

Let's see what the di-rectal diameter is and why its detection is important. 

In this article

  • BDP fetus, what is it
  • BDP, because it matters 
Read also: Fetal weight: growth charts

BPD or biparietal diameter, what is it

La Del Paesena Society of Obstetric Gynecological Ultrasound defined, in the Guidelines (2022), the BPD (BiParietal Diameter) parameter as follows: «biparietal diameter of the fetal head».

This is a parameter detected - by ultrasound - after the twelfth week of pregnancy and which specifically indicates the distance between the baby's two parietal bones. In simpler terms, it is the measurement of the distance between the baby's two ears.  

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

BDP, because it is important to note this parameter

The detection of this parameter is necessary to verify the correct growth of the fetus. Furthermore, it allows to estimate - with an accuracy of about 3 or 4 days - thegestational age in the period from the twelfth to the sixteenth week. 

You can know the parameter BPD of the fetus by ultrasound. In particular, by comparing with some standard reference values ​​(for each phase of pregnancy) it is possible to establish whether the distance between the two parietal bones is normal or not.

The BPD parameter is also needed to get a pixy Of weight of the fetus, another fundamental data to understand if the development of the baby in the belly is normal. 

The gynecologist Marinella Dell'Avanzo, expert in obstetric ultrasounds of Humanitas San Pio X, explained that through the formula in Hadlock fetal weight can be calculated based on a combination of parameters:

  • the biparietal diameter (or BPD);
  • the head circumference (CC);
  • the abdominal circumference (CA);
  • 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", he precised.

Finally, the calculation of the BPD parameter contributes to the final evaluation, by gynecological specialists, of the data detected by the ultrasound machines on the growth curves or tables. Biparietal diameter (BPD), head circumference (CC), abdominal circumference (CA), femur (FL) and fetal weight are positioned on the percentiles correspondents to understand if the fetus is growing correctly.

Finally, all clinical evaluations on the data obtained from the ultrasound are to be done only with the gynecologist specialist. Dr. Dell'Avanzo specifies: «We must not be frightened if the data reported on the growth curves they are not in 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 ".

Sources for this article: 

SIEOG Guidelines

Plos Medicine

  • pregnancy ultrasound scans
  • fetus weight calculation
  • growth curves
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