Since the biparietal diameter can be affected by different fetal head shapes, the head circumference can useful for providing a comparison with femur length.
femur length to head circumference ratio =
= (femur length in cm) / (head circumference in cm)
The normal ratio predicted from the menstrual age:
femur length-to-head circumference ratio =
= 0.138 + (0.00216 * (menstrual age in weeks))
where:
• The standard deviation is 0.01 to 0.02
Interpretation:
• A normal ratio exclude dwarfism.
• A low ratio suggests possible dwarfism.
• A high ratio suggests possible microcephaly.
Limitations:
• The ratio will not be affected by growth abnormalities that do not affect the head or long bones.
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