Isolated Hypoplastic or Absent Nasal Bone
When the baby's nose bone looks very small or is not seen
The nasal bone is a small bone in the baby's nose. On ultrasound, it may look smaller than usual or may not be seen.
Hypoplastic means smaller than usual. Absent means it is not seen. Isolated means it is the only finding.
This finding can raise the chance of Down syndrome, but it does not make a diagnosis by itself.
What happens first?
- A detailed ultrasound checks for any other findings.
- If no other findings are seen, it is called isolated.
- Your care team reviews your earlier test results.
No earlier screening
- The Society for Maternal‑Fetal Medicine (SMFM) recommends families should be offered counseling to help them understand the chance of trisomy 21 and to talk about the different choices they have.
- Cell-free DNA screening may be offered.
- If needed, a quad screen or amniocentesis may be discussed.
Low-risk blood screening
- The SMFM recommends counseling to help understand the chance of trisomy 21 and a discussion of options
- Choices may include no more testing, cell-free DNA, or amniocentesis.
Low-risk cell-free DNA
- For pregnant people who have a negative cfDNA screening and an isolated finding of absent or very small nasal bone, the SMFM recommends no more testing for chromosome problems.
Important to remember
- This is called a soft marker.
- It can raise risk, but it does not prove a diagnosis.
- Your care team will help you choose the next step.
Educational use only. This handout does not replace advice from your obstetrician or maternal-fetal medicine specialist. Updated July 17, 2026.
References
- Society for Maternal-Fetal Medicine. Consult Series #57: Evaluation and management of isolated soft ultrasound markers for aneuploidy in the second trimester. Am J Obstet Gynecol. 2021;225:B2-B15.
- American College of Obstetricians and Gynecologists. Practice Bulletin No. 226: Screening for Fetal Chromosomal Abnormalities. Obstet Gynecol. 2020;136:e48-e69.