Ultrasonographic prediction of fetal outcome in suspected skeletal dysplasias with use of the femur length-to-abdominal circumference ratio

OBJECTIVE: Our purpose was to determine whether the femur length-to-abdominal circumference ratio can be used antenatally to predict a lethal skeletal dysplasia. STUDY DESIGN: All obstetric sonograms performed from January 1990 to October 1995 were reviewed (44,020 studies) to find those scans suggestive of a skeletal dysplasia. Thirty patients were identified. The femur length/abdominal circumference ratio was then calculated from each patient's initial and subsequent sonograms. Birth outcomes were obtained on the 27 patients who elected to continue their pregnancies. RESULTS: All fetuses with a lethal skeletal dysplasia (n = 12) had a ratio <0.16. The fetuses with a nonlethal dysplasia (n = 8) had ratios between 0.134 and 0.193, with only 1 fetus with a ratio <0.16. All fetuses with no evidence of a skeletal dysplasia after birth (n = 7) had femur length/abdominal circumference ratios >0.18. The 1 fetus with a ratio <0.16 who survived the neonatal period had extreme bowing and demonstrates the limitation of the ratio when bowing is present. CONCLUSIONS: A stillbirth or neonatal death occurred in 12 of 13 patients with a femur length/abdominal circumference ratio <0.16, independent of gestational age. Conversely, no fetus with a ratio >0.16 was found to have a lethal skeletal dysplasia. This information may be useful in counseling women when ultrasonography suggests the diagnosis of a skeletal dysplasia.