Title Isolated Absent or Hypoplastic Nasal Bone during Second Trimester Scan- Should it be Considered a Major Soft Marker?

Background: For many years, absent or hypoplastic nasal bone during prenatal scans has been reported to be associated with increased risk of chromosomal abnormalities. We aimed to study the association of isolated absent or hypoplastic nasal bone with chromosomal abnormalities and genetic syndromes. Methods: a retrospective observational study conducted at BLK Max foetal medicine unit, in which women with absent or hypoplastic nasal bone during second trimester scan between 16-24weeks of gestation were recruited over a period of 2 years, from June 2021 to June 2023. Nasal bone was defined as absent when no calcification was seen below the skin of the nasal bridge in mid sagittal view of facial profile, while defined as hypoplastic nasal bone when nasal bone length was <5th centile for gestation according to gestation corrected Indian charts. Absent or hypoplastic nasal bone was defined as isolated - when there was absence of other second trimester soft markers, no other structural abnormalities were seen on the ultrasound and biochemistry screen was low risk. Results :A total of 3008 women were scanned during this period, among these 21 (0.69%) foetuses were found to have absent or hypoplastic nasal bone during second trimester scan. Thirteen (61.9%) fetuses were defined to have isolated absent (n-9) or hypoplastic (n-4) nasal bone which were included in the final analysis. Out of 13, 11 women opted for invasive testing where fluorescence in situ hybridisation (FISH) for 5 common aneuploidies and Microarray (750K) were sent on the amniotic fluid as a unit protocol, and 2 parents opted against any further genetic testing. In all tested cases, FISH and chromosomal microarray reports were reported as normal. All pregnancies were continued in our study group and had favourable foetal outcomes.