Isolated short femur length on second-trimester sonography: a marker for fetal growth restriction and other adverse perinatal outcomes.

OBJECTIVES To estimate the association between isolated second-trimester short femur length and fetal growth restriction as well as other adverse perinatal outcomes. METHODS We conducted a retrospective cohort study of patients with singleton gestations presenting for sonography between 16 and 24 weeks' gestation from 1990 to 2009. Cases of aneuploidy, skeletal dysplasia, and major anomalies were excluded. Short femur length was defined as length below the 10th percentile for gestational age and was considered isolated when both the estimated fetal weight and abdominal circumference were above the 10th percentile for gestational age. Isolated short femur length below the 5th percentile was also evaluated. The primary outcome was fetal growth restriction, defined as birth weight below the 10th percentile. Secondary outcomes included preeclampsia and preterm birth before 37 and 34 weeks. Univariable and multivariable logistic regression analyses were used to estimate the risk of these outcomes in fetuses with isolated short femur length. RESULTS Of 73,884 patients, 569 (0.8%) had a fetus with a femur length below the 10th percentile, of which 268 (47.1%) were isolated; 210 patients (0.3%) had a fetus with a femur length below the 5th percentile, of which 34 (16.2%) were isolated. Isolated short femur lengths below the 10th and 5th percentiles were associated with an increased risk of fetal growth restriction (<10th: adjusted odds ratio [aOR], 3.4; 95% confidence interval [CI], 2.4-4.6; <5th: aOR, 4.6; 95% CI, 2.0-10.7) and also with an increased risk of preterm birth before 37 and 34 weeks. There was no significant association between isolated short femur length and preeclampsia. CONCLUSIONS Isolated short femur length on second-trimester sonography is associated with a greater than 3-fold increased risk of fetal growth restriction and an increased risk of preterm birth. Serial growth assessment may be warranted in these cases.