Biparietal diameter

Last revised by Wilson Tao on 9 Oct 2024

Biparietal diameter (BPD) is one of the basic biometric parameters used to assess fetal size.  

BPD together with head circumference (HC), abdominal circumference (AC), and femur length (FL) are computed to produce an estimate of fetal weight.  In the second trimester this may be extrapolated to an estimate of gestational age and an estimated due date (EDD).

Measurement technique

The BPD should be measured on an axial plane that traverses the thalami, and cavum septum pellucidum. The transducer must be perpendicular to the central axis of the head, and thus the hemispheres and calvaria should appear symmetric. 

The calipers can be placed at the:

  • outer-to-inner edge of the calvarial wall

  • outer-to-outer edge of the calvarial wall (preferred) 5

The cerebellar hemispheres should not be in the plane of the image. 

Interpretation

BPD has been shown to be accurate in predicting gestational age from 14 to 20 weeks 2. The variability increases after this time.  BPD may also be influenced by factors such as abnormalities of head shape, breech presentation, or multiple gestations. Head circumference (HC) may be a more reliable measurement if there is variant head shape 4.

See also

Updating… Please wait.

 Unable to process the form. Check for errors and try again.

 Thank you for updating your details.