Is a short femur length a useful ultrasound marker for Down's syndrome?

To assess the significance of a short femur in the antenatal diagnosis of Down's syndrome a retrospective analysis was made of all Down's syndrome fetuses diagnosed over a 3 year period. 11 fetuses were found to have had a biparietal diameter and femur length measurement taken between 15 and 24 weeks gestation. Normal graphs were plotted for femur length against gestational age, femur length against biparietal diameter and biparietal diameter/femur length against gestational age. When the Down's syndrome measurements were plotted on the normal graphs there were four abnormal measurements. These four measurements, however, represented two fetuses both of which demonstrated other abnormalities, one cystic hygroma and one duodenal atresia with choroid plexus cysts. We do not feel that the femur length measurement is useful in the antenatal diagnosis of Down's syndrome.