Outcome of fetuses with short femur length detected at second‐trimester anomaly scan: a national survey

Objective To assess the relationship between the finding of fetal femur diaphysis length ( FL ) below the 5 th percentile at the second‐trimester scan and pregnancy outcome, in a population in which more than 90% of women attend first‐trimester screening. Methods This was a retrospective study of all Danish singleton pregnancies with a 17–22‐week anomaly scan between 1 January 2008 and 30 June 2011. Information on FL and gestational age ( GA ) at anomaly scan, on birth weight and GA at delivery and on chromosomal abnormalities was obtained from the Danish Fetal Medicine Database. Results Short FL was identified in 2718 (1.8%) of 147 766 fetuses and was present in 11 (16.2%) of the 68 fetuses affected by trisomy 21 (positive likelihood ratio ( LR +) 8.8 (95% CI , 5.1–15.2)). Trisomy 13/18 and unbalanced autosomal structural abnormalities were also associated with a short FL in three (12.0%, LR + 6.5 (95% CI , 2.3–18.9)) and eight (32.0%, LR + 17.4 (95% CI , 9.8–30.9)) of the cases, respectively. The risk of a fetus having trisomy 21, trisomy 18, trisomy 13 or an unbalanced autosomal structural abnormality was 1 : 123 (95% CI , 79–192), given a short FL . Pregnancies with a fetus with short FL were more often affected by early preterm delivery (before 34 weeks) (5.6%; odds ratio ( OR ) = 4.2 (95% CI , 3.5–4.9)) and small‐for‐gestational‐age ( SGA ) infants (13.9%; OR = 4.3 (95% CI , 3.8–4.8)). Conclusion Short FL at the second‐trimester anomaly scan is associated with a significantly higher relative risk of chromosomal abnormalities, and a substantially higher absolute risk for SGA and early preterm delivery. Copyright © 2013 ISUOG. Published by John Wiley & Sons Ltd