Concerns Over Short Nasal Bone Measurement in 21-Week Ultrasound

High-resolution ultrasound examination shows a shorter nasal bone?

During the 21-week ultrasound, the high-level ultrasound technician noted that the nasal bone measurement was approximately 5.7 cm, which is below the standard value of 6 cm, indicating a risk of Down syndrome that is over 13 times higher.
However, we have previously undergone early screening for Down syndrome through blood tests and ultrasound, which returned normal results (1st Trimester Risk 1:14483, Trisomy 18 risk 1:>20000, Trisomy 13 risk 1:>20000).
Given this information, do we need to consider amniocentesis or other tests? We are really worried and would like to seek the physician's opinion.
Thank you!

Niming, 30~39 year old female. Ask Date: 2015/12/01

Dr. Lin Wenbin reply Obstetrics and Gynecology

All non-invasive tests (such as ultrasound and blood tests) cannot definitively determine whether a baby has Down syndrome; they can only provide a ratio or probability.
A high probability does not mean the baby will definitely have Down syndrome, and a low probability does not mean the baby will definitely not have it.
Based on your situation, the first-trimester screening results appear to be low risk.
The high-level ultrasound shows a shorter nasal bone, but if no other associated abnormalities are observed, the risk is not particularly high, and further testing may not be necessary.
In the Asian population, the incidence of underdeveloped or shorter nasal bones in normal babies is not low.
However, if you are very concerned about having a baby with Down syndrome and need 100% confirmation, you may need to undergo amniocentesis to obtain a definitive answer.
If you are worried about the risks of amniocentesis, you might consider non-invasive prenatal testing (NIPT/NIFTY/NIPS), which is more expensive but has a sensitivity of up to 99% for detecting Down syndrome (the first-trimester combined screening with nuchal translucency and blood tests has a detection rate of about 90%).
Although it cannot provide a 100% guarantee, this is currently the most sensitive non-invasive test available.
I hope this helps clarify your concerns.
(Response from Dr.
Tian Yi-wen, Department of Obstetrics and Gynecology, Changhua Hospital)

Reply Date: 2015/12/01

More Info

Concerns regarding the measurement of the nasal bone during a 21-week ultrasound can understandably cause anxiety for expectant parents. The nasal bone is an important marker that can be assessed during a routine ultrasound, and its measurement can sometimes be associated with certain chromosomal abnormalities, including Down syndrome (Trisomy 21). In your case, the measurement of 5.7 mm, which is below the standard value of 6 mm, has raised concerns due to the reported increased risk of Down syndrome.

However, it is crucial to interpret these findings in the context of the overall risk assessment. You mentioned that earlier screenings, including blood tests and ultrasounds, indicated a very low risk for Down syndrome and other trisomies (Trisomy 18 and Trisomy 13). Specifically, a risk of 1 in 14,483 for Down syndrome is considered quite low, and the risks for Trisomy 18 and Trisomy 13 are even lower than that. These results suggest that the likelihood of these conditions is minimal.

In light of this information, the decision to pursue further testing, such as amniocentesis, should be made carefully. Amniocentesis is an invasive procedure that carries its own risks, including miscarriage. It is typically recommended when there is a significant concern for chromosomal abnormalities, especially if non-invasive screening tests indicate a higher risk.

Given your previous low-risk screening results, it may be beneficial to discuss your concerns with your healthcare provider. They can provide a more comprehensive evaluation of your situation, considering both the nasal bone measurement and the results of your earlier tests. They may also suggest additional non-invasive testing options, such as cell-free fetal DNA testing, which can provide more information about the risk of chromosomal abnormalities without the risks associated with invasive procedures.

It is also important to remember that variations in nasal bone length can occur for a variety of reasons, and not all cases of a shorter nasal bone lead to chromosomal abnormalities. Your healthcare provider can help you understand the implications of the ultrasound findings and guide you in making an informed decision about whether further testing is necessary.

In summary, while the measurement of the nasal bone is a factor to consider, it should be evaluated alongside your overall risk profile. Engaging in a detailed discussion with your healthcare provider will help clarify the best course of action for you and your baby, alleviating some of the anxiety you may be feeling. Remember, it is essential to prioritize your mental well-being during this time, and seeking support from healthcare professionals can be invaluable.

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