Overview

A short femur roughly measures at least one week behind all other biometric parameters in the second trimester. In the medical literature, a short femur is technically defined as being present when the actual femur length is < 0.91 times the expected value, or the biparietal diameter/femur length ration is > 1.5 SD above the mean. The data regarding the efficacy of femur length ratios for the detection of Down syndrome differs significantly among studies, and there is considerable overlap between affected and control pregnancies.

Given the inconsistency of the data regarding femur length as a predictor for Down syndrome, attempts have been made to use other measurement ratios, including humerus length, humerus plus femur length, humerus and/or femur length versus foot length, again with conflicting results.

The variation among studies may be attributed to differences in ultrasound equipment and technique between centers, differences in gestational ages of pregnancies studied, the tendency to disregard femur length or biparietal diameter measurements that appear inconsistent with the gestational age, or racial variability of the population served.

Recommendations

The finding of short femur and/or short humerus should prompt a search for additional markers for Down syndrome. Given the isolated finding of short femur and/or humerus, we recommend offering genetic counseling.

References

• 1. Benacerraf, BR, Gelman, R, and Frigoletto, FD. Sonographic identification of second-trimester fetuses with Down's syndrome. NEJM 1987; 317(22):1371-1376.
• 2. Lockwood, C, Benacerraf, B, Krinsky, A, Blakemore, K, Belanger, K, Mahoney, M, and Hobbins, J. A sonographic screening method for Down syndrome. Am J Obstet Gynecol 1987; 157:803-8.
• 3. Campbell WA, Vintzileos, AM, Rodis, JF, Ciarleglio, L, and Craffey, Alicia. Efficacy of the biparietal diameter/femur length ratio to detect Down syndrome in patients with an abnormal biochemical screen. Fetal Diagn Ther 1994; 9:175-182.
• 4. Platt, LD, Medearis, AL, Carlson, DE, Falk, RE, DeVore, GR, Horenstein J and Walla, CA. Screening for Down syndrome with the femur length/biparietal diameter ratio: A new twist of the data. Am J Obstet Gynecol 1992; 167(1):124-128.
• 5. Twining, P, Whalley, DR, Lewin E, and Foulkes, K. Is a short femur length a useful ultrasound marker for Down's syndrome. British Journal of Radiology 1991; 64: 990-992.
• 6. Johnson, MP, Michaelson, JE, Barr, M, Treadwell, MC, Hume, RF, Dombrowski, MP and Evans, MI. Combining humerus and femur length for improved ultrasonographic identification of pregnancies at increased risk for trisomy 21. Am J Obstet Gynecol 1995; 172:1229-35.
• 7. Rodis, JF, Vintzileos, AM, Fleming, AD, Ciarleglio, L, Nardi, DA, Feeney L, Scorza, WE, Campbell, WA and Ingardia, C. Comparison of humerus length with femur length in fetuses with Down syndrome. Am J Obstet Gynecol 1991; 165:1051-1056.
• 8. Biagiotti, R, Periti, E and Cariati, E. Humerus and femur length in fetuses with Down syndrome. Prenatal diagnosis 1994; 14:429-434.
• 9. Johnson, MP, Barr, M, Treadwell, MC, Michaelson, J, Isada, NB, Pryde, PG, Dombrowski, MP, Cotton, DB, and Evans, MI. Fetal leg and femur/foot length ratio: A marker for trisomy 21. Am J Obstet Gynecol 1993; 169:557-63.
• 10. Grandjean, H and Sarramon, MF. Femur/foot length ratio for detection of Down syndrome: Results of a multicenter prospective study. Am J Obstet Gynecol 1995; 173:16-19.
• 11. Johnson, MP, Michaelson, JE, Barr, M, Treadwell, MC, Isada, NB, Dombrowski, MP, Pryde, PG, Addis, J and Evans, MI. Sonographic screening for trisomy 21: Fetal humerus:foot length ratio, a useful new marker. Fetal Diagn Ther 1994; 9:130-138.