The cephalic index (CI) is a ratio/value calculated using two fetal biometric parameters which are the occipitofrontal diameter (OFD) and the biparietal diameter (BPD). It is measured with the cursor placed on the outer edge to the outer edge of the cranial bone.
It is calculated as:
cephalic index (CI) = biparietal diameter (BPD)/occipitofrontal diameter (OFD) x 100
The cephalic index gives an idea of the fetal head shape. It can change according to various situations such as
presentation, e.g. breech presentation
ruptured membranes
presence of a twin pregnancy
The usual range is variable depending on various sources and different demographic groups. Often the mean value is taken ~78 (range 74-83 3)
A grossly decreased cephalic index suggests dolichocephaly while a grossly increased one can suggest brachycephaly.