Revisiting absent nasal bone in the second trimester.

Area of Science:

• Prenatal diagnosis
• Fetal medicine
• Genetics

Background:

• The nasal bone is a common marker evaluated during second-trimester ultrasounds.
• Absent nasal bone has been associated with an increased risk of aneuploidy, particularly Down syndrome (trisomy 21).
• Accurate risk assessment is crucial for appropriate genetic counseling and management.

Purpose of the Study:

• To assess the outcomes of fetuses diagnosed with an absent nasal bone in the second trimester.
• To determine the association between isolated absent nasal bone and aneuploidy.
• To evaluate the predictive value of absent nasal bone in conjunction with other screening parameters.

Main Methods:

• Prospective, observational study of fetuses with absent nasal bone from 16 weeks gestation.
• Inclusion of fetuses diagnosed at or referred to a fetal medicine center.
• Offer of amniocentesis for fetal karyotype and microarray; follow-up for those declining invasive testing.

Main Results:

• 26 fetuses were included in the study.
• Aneuploidy was diagnosed in 8 (30.8%) fetuses: 7 with trisomy 21 and 1 with trisomy 18.
• All aneuploid fetuses exhibited additional ultrasound abnormalities and/or high-risk biochemical screening results.

Conclusions:

• Isolated absent nasal bone in the second trimester, with prior low-risk combined screening by certified sonographers, is unlikely to be associated with Down syndrome.
• The presence of additional sonographic markers or elevated biochemical risk significantly increases the likelihood of aneuploidy in fetuses with absent nasal bone.
• This finding supports a refined approach to genetic counseling and invasive testing recommendations based on the complete prenatal screening profile.