EP18.10: Isolated unossified nasal bone in the first trimester: does it really improve Trisomy 21 detection? Experiences from Southern India

Large scale applicability of nasal bone (NB) as a screening tool for trisomy 21 (T21) has been difficult in low-risk populations. We assessed the additive value of first trimester NB assessment to detect T21 in our population. This retrospective analysis included all structurally normal fetuses where NB was assessed, nuchal translucency (NT) was <3.5mm and outcomes were available; over 5 years in a tertiary fetal medicine centre. First trimester combined screening (FTS) results were documented with a cut-off of ≥ 1:250 as high-risk. Percentage of fetuses with or without unossified nasal bone (UNB), fetuses with UNB with or without other aneuploidy markers, and T21 fetuses in each group were determined. The number of invasive procedures required to pick up one T21 fetus in the cohort with UNB was also evaluated. We included 2,620 structurally normal fetuses, of which 55 had T21. 45.7% of the total study group had UNB. The highest percentage of T21 was seen when UNB was associated with NT >3mm (33.3%) or with other aneuploidy markers (26.7%). Only 1.6% fetuses with an isolated UNB had T21. 1.9% fetuses with UNB and FTS positive result had T21, and only 0.3% of fetuses with UNB and FTS negative had T21. This percentage was no different than in the group with NB present (FTS Positive – 1.5% T21, FTS Negative – 0.3% T21). The pick up of T21 in the isolated UNB group was 1 per 61 invasives. If maternal age was <35 years, the pick-up would be 1 per 81. When isolated UNB had an FTS positive result, the pick up of T21 was 1 per 53. In isolated UNB with FTS negative result, only 1 T21 would be picked up per 290 procedures, similar to that in FTS negative fetuses with NB present and no aneuploidy markers (1 in 319). Isolated first trimester UNB does not increase T21 risk significantly in our population. This is even lesser with low risk FTS. The number of invasives needed to pick up one T21 in the isolated UNB cohort is much higher (1 per 53) than that needed for FTS positive (1 per 25).