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Biparietal Diameter to Femur Length Ratio

Description

The ratio of biparietal diameter to femur length at 15 to 23 weeks of gestation (menstrual age) can be used to identify fetuses at risk for Down syndrome. Infants with Down syndrome have shortening of the femur, which contributes to an increase in the ratio above normal.


 

biparietal diameter to femur length ratio =

= (biparietal diameter in cm) / (femur length in cm)

 

predicted ratio for menstrual age =

= 3.222 - (0.125 * (menstrual age in weeks)) + (0.002 * ((menstrual age in weeks) ^2))

 

Interpretation:

• The ratio decreases with gestational age.

• Infants with Down syndrome have a ratio greater than 1.5 standard deviations above the mean for the control population.

 


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Specialty

ICD-10