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))
• The standard deviation is 0.01 to 0.02
• A normal ratio exclude dwarfism.
• A low ratio suggests possible dwarfism.
• A high ratio suggests possible microcephaly.
• The ratio will not be affected by growth abnormalities that do not affect the head or long bones.
To read more or access our algorithms and calculators, please log in or register.
Specialty: Obstetrics & Gynecology, Pedatrics