Head circumference, abbreviated HC, is one of the four standard biometric measurements obtained during obstetric ultrasound. It is measured at the same level as the biparietal diameter, where the thalami and cavum septum pellucidum are visible, by tracing an ellipse around the outer edge of the fetal skull. Head circumference is used in combination with other measurements to estimate gestational age and fetal weight, and it is particularly important for detecting abnormalities of head size. A head circumference below the expected range may suggest microcephaly, while one above the expected range may indicate hydrocephalus or macrocephaly.
Sonography Term