How to: Cephalic Index Measurements

The cephalic index is a measurement that helps to classify the size and shape of the fetal head. A health-care professional calculates the index by dividing the biparietal diameter of the head by the fronto-occipital diameter of the head and multiplying by 100. (A technician first measures these diameters by ultrasound.) Normal cephalic index measurements range from 74 to 83. A cephalic index measurement less than 74 indicates scaphocephaly, when the head is longer than expected relative to the width; a measurement greater than 83 indicates brachycephaly, when the head is shorter than expected relative to the width.

Things You'll Need

• Abdominal ultrasound apparatus

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Instructions

• 1

  Find the biparietal diameter of the fetal head by obtaining a transverse view by ultrasound at the level of the cavum septum pelucidum. Freeze the screen on that view. Use the caliper function on the ultrasound to measure the distance from the outer edge of the parietal bone on one side of the skull to the inner edge of the parietal bone on the other side of the skull.

• 2

  Determine the fronto-occipital diameter of the fetal head using the same screen shot you used to find the biparietal diameter. Use the caliper function to measure the distance from the outer edge of the frontal bone to the inner edge of the occipital bone.

• 3

  Divide the biparietal diameter by the fronto-occipital diameter and multiply by 100 to determine the cephalic index.