Femur length parameters in fetuses with Down syndrome†

Objective: To characterize the natural history of femur length (FL) parameters across gestation in Down syndrome fetuses. Methods: Retrospective review of singletons with fetal Down syndrome delivered at our institution between January 2009 and December 2015. We assessed FL <3rd percentile for gestational age, femur length/abdominal circumference (FL/AC) ratio below 20%, and FL lag, defined as difference in weeks between obstetric gestational age and gestational age corresponding to measured FL. Ultrasound data were grouped into 6-week intervals, with initial examination at each interval selected for analysis. Relationship between FL parameters and small for gestational age (SGA) infant was evaluated. Results: During the study period, 173 pregnancies with fetal Down syndrome had 310 sonograms and subsequent delivery. Prior to 22 weeks, FL <3rd percentile occurred in 38 (26%), and FL/AC ratio below 20% occurred in 73 (51%). At each ultrasound interval, FL/AC ratio below 20% was more prevalent than FL <3rd percentile (all p<.05). Proportion with FL and FL/AC below these thresholds did not vary across gestation (all p>.05). However, FL lag increased from <1 week in the early second trimester to almost 3 weeks ≥ 34 weeks (p<.001). SGA Down syndrome infants were not more likely to have FL <3rd percentile or FL/AC ratio below 20% than their non-SGA counterparts. Conclusion: Among Down syndrome fetuses, FL/AC ratio below 20% was approximately twice as common as FL <3rd percentile, regardless of gestational age. Neither parameter was associated with SGA. With advancing gestation, degree of FL lag progressively increased.