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Head circumference (HC) is one of the basic biometric parameters used to assess fetal size. HC together with biparietal diameter (BPD), 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).
- HC is measured on the same plane as BPD, that is on an axial plane that traverses the thalami and cavum septum pellucidum.
- transducer must be perpendicular to the central axis of the head, and thus the hemispheres and calvaria should appear symmetric.
- cerebellar hemispheres should not be in the plane of the image, or the probe is too caudal giving an inaccurate size of the fetal head.
If the available ultrasound equipment allows for measurement of an ellipse, this may be drawn around the outside of the calvarium. Alternatively, HC may be calculated from BPD and occipitofrontal diameter (OFD) as: HC = 1.62 × (BPD + OFD) 3.
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