OC040: Absent nasal bone at 11-14 weeks of gestation and chromosomal defects

This study examines the association between absent nasal bone (NB) at the 11–14 weeks scan and chromosomal defects. Ultrasound examination of the fetal profile was carried out and the presence or absence of the NB was noted immediately before karyotyping in 3829 fetuses at 11–14 weeks. The fetal profile was successfully examined in 3788 (98.9%) cases. In 3358 of the 3788 cases the fetal karyotype was normal and in 430 it was abnormal. In the chromosomally normal group the incidence of absent NB was related firstly, to the ethnic origin of the mother, being 2.5% for Caucasians, 10.4% for Afro-Caribbeans and 6.8% for Asians, secondly to fetal crown-rump length (CRL), being 4.6% for CRL of 45–54 mm, 3.9% for CRL of 50–64 mm, 1.5% for CRL of 65–74 mm and 1% for CRL of 75–84 mm and thirdly, to nuchal translucency (NT) thickness, being 2% for NT < 2.5 mm, 3% for NT 2.5–3.4, 5% for NT 3.5–4.4 and 12% for NT > 4.5. In the chromosomally abnormal group there was absent NB in 161 of the 242 (67%) with trisomy 21, in 48 of the 84 (57%) with trisomy 18, in 7 of the 22 (32%) with trisomy 13, in 3 of the 34 (9%) with Turner syndrome and in 4 of the 48 (8%) with other defects. At the 11–14 weeks scan the incidence of absent NB is related to the presence or absence of chromosomal defects, CRL, NT thickness and ethnic origin.