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OC9.01: Selective fetocide in monochorionic twins

Objective: Our objective is to evaluate the efficiency of BPD/NB ratio as an independent marker for fetal aneuploidy. Methods: Prospective cohort study of women presenting for anatomic survey between 15 and 22 weeks of gestation. The fetal nasal bone (NB) and other markers of fetal aneuploidy including: nuchal fold, femur (FL) and humeral (HL) lengths, choroid plexus cysts, major fetal anomaly, echogenic bowel, pyelectasis and hypoplastic fifth digits, were evaluated. NB hypoplasia was defined as a BPD/NB ratio > 11. Short FL and HL were defined as measurements < 0.91 and 0.89 compared to the expected measurements respectively. Fetuses with fetal aneuploidy were compared with those without for the presence of NB hypoplasia either as an isolated marker or in the presence of other markers. Results: Of 2885 women evaluated, NB measurements were obtained in 2465 (85%). There were 35 (1.4%) cases with fetal aneuploidy (22 with trisomy 21, 7 trisomy 18, 2 Turner’s syndrome, 1 triploidy and 3 Marker chromosome). The mean maternal age was 30.2 ± 6.7 years and 50% of the study population were white, 40% black, 6% asian and 4% others. The sensitivity, specificity, positive and negative predictive values of using NB as an isolated marker were 43%, 85%, 4% and 99%, respectively, compared with 61%, 76%, 3.6% and 99%, when there were other markers present. The likelihood ratios of positive and negative tests were: 2.9 and 0.87 respectively for isolated NB hypoplasia; and 2.7 and 0.7 respectively when NB is combined with other markers. The sensitivity, specificity, positive and negative predictive values for trisomy 21 as an isolated marker were 57%, 85%, 3%, and 99% respectively, with an LR positive of 3.8; and with other markers: 73%, 85%, 4% and 99%, respectively, with LR+ of 4.9. Conclusions: The BPD/NB ratio > 11.0 is a marker for fetal aneuploidy. However, as an isolated marker, the efficiency of nasal bone hypoplasia as an isolated marker is modest.