The head circumference to abdominal circumference ratio is useful in detecting asymmetrical growth retardation.


head circumference to abdominal circumference ratio =

= (head circumference in cm) / (abdominal circumference in cm)



• The ratio varies over the gestational period: normal at 24 weeks is 1.12, at 32 weeks is 1.05 and at 40 weeks is 0.98.


An equation that predicts the ratio from the menstrual age is:


HC-to-AC ratio =

= 1.32293 - (0.0084471 * (menstrual age in weeks))


but the R^2 value is low.



• The change in the ratio over gestation varies considerably between individuals.

• The ratio can be abnormal even when the individual circumferences are not abnormal; thus an abnormal ratio should only be held significant if either the head circumference, abdominal circumference or both are abnormal.


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