Short femur-below the fifth centile for gestational age might reflect inaccurate dating or may be a normal variant in a constitutionally small fetus, but it also has been associated with skeletal dysplasias and chromosomal abnormalities. Recent studies have reported an association between isolated short femur in the second trimester and delivery of a small-for-gestational age infant, possibly as the result of uteroplacental insufficiency. This retrospective study reviewed all pregnancies seen at a tertiary referral unit at 18-24 weeks' gestation with a fetal femur length below the fifth centile for gestational age. The gestational age and the size of the fetal nuchal translucency had been determined as part of a Down syndrome screening test at 11-14 weeks' gestation. Repeated fetal measurements identified 11 fetuses who did not have short femur. Of the 118 remaining cases, detailed ultrasonographic examination demonstrated associated abnormalities in 46, or 39%. Among these cases, the associated conditions were skeletal dysplasia (35%), chromosomal abnormalities (22%), and genetic syndromes (9%). In 35% of cases, the fetus had multiple anomalies without a specific diagnosis. In 72 instances, 61% of the total, short femur was an isolated finding. In 46% of isolated cases, evaluation revealed early, severe intrauterine growth restriction with abnormal umbilical artery Doppler findings, and delivery took place before 37 weeks' gestation. In 90% of these cases, abnormal uterine artery Doppler findings were apparent at the time of presentation. Preeclampsia developed in 36% of these pregnancies, and the rate of intrauterine death was 33%. Both of these events were considerably less frequent when uterine artery Doppler imaging was normal. In pregnancies with short fetal femur, the major determinants of outcome are the presence of other fetal abnormalities and abnormal uterine artery Doppler findings.