The yield of CMA compared to the background risk
We assessed the yield of CMA in pregnancies complicated with short long bones compared to 2 cohorts that represented the background risk: a cohort of 5,541 uncomplicated pregnancies for which the yield of CMA was 1.4%13 and a cohort of 10,614 cases extrapolated from a meta-analysis by Srebniak et al.,14 for which the yield of CMA was 1.1%. The rate of chromosomal abnormalities was significantly higher for all short long bones cases as compared to the background population for both cohorts: 4.5 (95% CI 1.6-12.7; P=0.0017) and 5.7 (95% CI 2-16.2; P<0.001). Furthermore, the yield for both isolated and non-isolated cases was significantly higher than the background risk (P<0.05). The yield was higher than background risk for cases diagnosed with short long bones after 22 weeks of gestation compared to both cohorts, but not for cases diagnosed after 24 weeks (Table 3). For cases in which the lowest estimated bone length percentile was above the 3rd percentile (but below the 5th percentile), the yield was also higher than background risk.