Objectives: Failure to visualise the fetal nasal bones at 11-14 weeks of gestation is associated with a significant increase in the risk for trisomy 21. However, it is not known whether the ethnic origin of the mother has any effect on the fetal profile and the prevalence of this marker.
Design: Prospective study.
Setting: London Teaching Hospital.
Population: Four thousand and four hundred and ninety-two consecutive fetuses undergoing routine first trimester ultrasound scanning at 11-14 weeks of gestation in a multiethnic population.
Methods: Examination of the nasal bones was attempted in the fetuses.
Main outcome measure: Rate of visualisation of the fetal nasal bones.
Results: Five hundred fetuses were excluded from the analysis because of chromosomal abnormalities or a technically unsatisfactory examination. In the remaining 3992 fetuses, the maternal ethnic origin was African in 13.0%, Asian in 15.3% and Caucasian in 66.0%. Compared with Caucasians, the failure to visualise the fetal nasal bones was significantly higher in women of African (P= 0.0001) but not Asian origin (P= 0.24). A multivariable logistic regression model showed that having a mother of African origin is still significantly associated with an increased likelihood of absent fetal nasal bones compared with Caucasians (odds ratio 2.33), even after correcting for maternal age, parity and crown-rump length.
Conclusion: There is a significant difference in the rate of visualisation of the fetal nasal bones in the first trimester in mothers of different ethnic origin. This suggest that corrections for maternal ethnicity will be required to ensure equity of fetal nasal bone screening in multiracial populations. Whether corrections are required for the father's ethnic origin remains to be determined.