Revision 11 for 'Biparietal diameter'

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Biparietal diameter

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 cavalria should appear symmetric. 

The calipers should be placed on the outer edge of the near cavalrial wall and inner edge of the far cavalrial wall.  


BPD has been shown to be accurate in predicting gestational age from 14 to 20 weeks2. The variability increases after this time.  BPD may also be influenced by factors such as abnormalitis of head shape, breech presentation, or multiple gestations.

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