A ratio of the cranium derived to determine the normality of the fetal head shape is called what?

• A. Biparietal diameter
• B. Cephalic index
• C. Occipitofrontal diameter
• D. Head circumference

Answer: (B)

The cephalic index is the ratio used to assess head shape. It compares the biparietal diameter (width of the head) to the occipitofrontal diameter (length from the frontal to occipital bones) and is calculated as BPD divided by OFD times 100. This index helps classify head shape as normal or dolichocephalic (long, narrow) or brachycephalic (short, broad). Normal ranges are typically around 75–85%, with higher values indicating a broader head and lower values indicating a longer, narrower head.

The other measurements are single dimensions rather than a ratio: biparietal diameter measures width, occipitofrontal diameter measures length, and head circumference reflects overall size. They don’t by themselves describe head shape, which is what the cephalic index assesses.