What is the Nasal Bone in Fetal Development?
The nasal bone is a small paired bone that forms the bridge of the nose. In fetal development, it begins to ossify (harden and become visible on ultrasound) between 11 and 14 weeks of gestation. A well-formed, clearly visible nasal bone is considered a normal finding during prenatal ultrasound and serves as an important soft marker when assessing fetal anatomy and chromosomal risk.
The nasal bone is not merely a structural feature — its presence, size, and clarity on ultrasound are used as part of a comprehensive first-trimester and second-trimester screening protocol. Understanding what the nasal bone represents and how it is evaluated is an important step for expectant parents navigating prenatal screening.
What Does Absent Nasal Bone Mean During Pregnancy?
An absent nasal bone means that the nasal bone is either not visible or cannot be detected on ultrasound at the time of the scan. This finding does not mean that the fetus will definitely have a chromosomal abnormality or a structural problem. It is classified as a soft marker — a feature that, on its own, may be a normal variant, but when combined with other findings, can indicate an increased risk of certain chromosomal conditions.
It is important to understand that absent nasal bone is a screening finding, not a diagnosis. Many babies with no visible nasal bone on first-trimester ultrasound go on to be born completely healthy with a normally formed nose. However, the finding does warrant a thorough evaluation by a fetal medicine specialist and may prompt further testing.
When is Nasal Bone Assessed in Pregnancy?
First Trimester Screening
The nasal bone is routinely assessed as part of the first-trimester combined screening, which takes place between 11 weeks and 13 weeks 6 days of gestation. During this scan — known as the NT (nuchal translucency) scan — a trained sonographer or fetal medicine specialist evaluates the nasal bone along with the nuchal translucency thickness, fetal heart rate, and maternal blood markers (beta-hCG and PAPP-A). Together, these parameters are used to calculate the overall risk of chromosomal abnormalities such as Down syndrome (trisomy 21), trisomy 18, and trisomy 13.
At this stage, a nasal bone is considered absent if not echogenic (bright we correct mid-sagittal view of the fetal profile. The assessment requires that the shite) line is visible in specific technical criteria to be met, including the correct angle of insonation, to avoid false-positive or false-negative findings.
Second Trimester Anomaly Scan
If the nasal bone was not assessed or was inconclusive in the first trimester, it can be re-evaluated during the mid-pregnancy anomaly scan, typically performed between 18 and 22 weeks. By this stage, the nasal bone should normally be clearly visible and measurable. Hypoplasia (an unusually small nasal bone) at this stage may also be noted as a soft marker for Down syndrome. The second-trimester scan also assesses for other structural anomalies that may accompany chromosomal conditions.
Common Causes of Absent Nasal Bone
Genetic and Chromosomal Factors
The most clinically significant association of absent nasal bone is with chromosomal abnormalities, particularly trisomy 21 (Down syndrome). Studies show that the nasal bone is absent in approximately 60 to 70 percent of fetuses with Down syndrome at the time of first-trimester screening, compared to only 1 to 3 percent of chromosomally normal fetuses. Absent nasal bone has also been reported in association with trisomy 18, trisomy 13, and Turner syndrome, though less frequently than with Down syndrome.
Ethnic and Developmental Variations
It is important to recognise that the prevalence of absent nasal bone varies significantly across ethnic groups. Studies indicate that absent nasal bone is more commonly seen in fetuses of Afro-Caribbean descent than in Caucasian or Asian populations, even in chromosomally normal pregnancies. This ethnic variability must be taken into account when interpreting the finding — what may be a significant marker in one population group carries a different prior probability in another. This is why isolated absent nasal bone in the absence of other markers requires careful contextual interpretation by a fetal medicine expert.
Absent Nasal Bone and Chromosomal Abnormalities
Association with Down Syndrome
The nasal bone is one of the most powerful ultrasound markers for Down syndrome available in the first trimester. When combined with nuchal translucency measurement and maternal blood markers, the addition of nasal bone assessment significantly improves the detection rate of Down syndrome while reducing false-positive rates. An absent nasal bone increases the likelihood ratio for Down syndrome substantially, and when found alongside an increased nuchal translucency or abnormal biochemistry, the cumulative risk can be high enough to recommend diagnostic testing.
It is equally important to note that a normal nasal bone does not exclude Down syndrome, as approximately 30 to 40 percent of fetuses with Down syndrome do have a visible nasal bone in the first trimester.
Risk of Other Genetic Conditions
While Down syndrome is the most common association, absent nasal bone can also be linked to other aneuploidies. Trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome) are associated with a wider range of structural anomalies, and absent nasal bone may be one feature in a broader pattern of findings. Less commonly, absent nasal bone has been reported in cases of skeletal dysplasias and certain congenital syndromes that affect craniofacial development. A fetal medicine specialist will evaluate the nasal bone finding in the context of the overall scan and risk profile.
How Nasal Bone iIs Evaluated on Ultrasound?
Accurate assessment of the nasal bone requires strict technical standards. The scan must be performed in a precise mid-sagittal view of the fetal face, with the ultrasound beam perpendicular to the nasal bone. At the correct angle, three distinct echogenic lines are normally seen: the skin overlying the nose, the nasal bone itself (which should be more echogenic than the overlying skin), and the tip of the nose. If only two lines are visible — the skin and the nasal tip — with no distinct, brighter nasal bone between them, it is classified as absent.
A falsely absent nasal bone can be reported if the angle of insonation is incorrect, if the image quality is suboptimal, or if the examination is performed too early in gestation. For this reason, nasal bone assessment should only be performed and interpreted by trained and accredited specialists. At Artemis Hospitals, Gurugram, fetal medicine consultants with FMF (Fetal Medicine Foundation) accreditation conduct these scans to the highest standards.
Is Absent Nasal Bone Always a Cause for Concern?
No — absent nasal bone in isolation is not always cause for significant concern. The interpretation depends on several factors: the gestational age at the time of scanning, the ethnic background of the mother, the presence or absence of other soft markers or structural anomalies, the nuchal translucency measurement, and the results of maternal blood screening.
When absent nasal bone is found as an isolated finding in a low-risk pregnancy with normal biochemistry, normal nuchal translucency, and no other markers, the overall residual risk of chromosomal abnormality may remain relatively low. In such cases, a repeat scan or non-invasive prenatal testing (NIPT) may be offered rather than immediate diagnostic testing. Conversely, when absent nasal bone is accompanied by other markers, the risk may be high enough to recommend diagnostic testing such as chorionic villus sampling (CVS) or amniocentesis.
Make Informed Decisions for Your Pregnancy
Consult a fetal medicine expert for detailed evaluation and pregnancy monitoring.
What are the Next Steps If Nasal Bone Is Absent?
Additional Screening Tests
If an absent nasal bone is detected, the first step is typically a detailed review of all first-trimester screening parameters by a fetal medicine specialist. If not already done, a combined first-trimester screen including blood markers can be arranged. Cell-free fetal DNA testing, commercially known as NIPT or NIPS, is a highly sensitive non-invasive test that analyses fetal DNA circulating in the maternal blood and can detect Down syndrome with greater than 99 percent sensitivity. NIPT does not carry any risk to the pregnancy and is often the preferred next step before considering invasive testing.
- Detailed Level II anomaly scan to assess for any structural abnormalities
- Repeat nasal bone assessment if the initial scan was technically suboptimal
- NIPT / cell-free fetal DNA testing for high-sensitivity chromosomal screening
- Genetic counselling to understand the results, risks, and options available
Diagnostic Tests During Pregnancy
If screening results or clinical assessment suggest a significantly elevated risk, diagnostic testing provides a definitive chromosomal diagnosis. These tests include:
- Chorionic Villus Sampling (CVS) — performed between 11 and 14 weeks, involves sampling placental tissue under ultrasound guidance. Results are available within one to two weeks and provide a definitive karyotype.
- Amniocentesis — performed after 15 weeks, involves sampling amniotic fluid. It carries a small procedure-related risk of pregnancy loss (approximately 0.5 to 1 percent) and provides a full chromosomal analysis.
Diagnostic testing is always voluntary and is offered with comprehensive genetic counselling so that families can make fully informed decisions.
How Artemis Hospitals Supports Advanced Prenatal Screening and Counseling?
Artemis Hospitals, Gurugram is home to a dedicated Fetal Medicine Unit that provides comprehensive prenatal screening, diagnosis, and genetic counselling under one roof. Our fetal medicine specialists are trained and accredited to international standards, including FMF certification, ensuring that all ultrasound assessments — including nasal bone evaluation — are performed with the highest level of precision and consistency.
Our approach to absent nasal bone and other prenatal findings is holistic and family-centred. We begin with a detailed clinical review and do not rush families into decisions. We offer integrated first-trimester screening, advanced NIPT options, expert level II ultrasound, and access to invasive diagnostic testing when needed — all supported by in-house genetic counselling.
For families receiving an unexpected finding, our team provides emotional support alongside clinical guidance, ensuring that parents understand their options clearly and have the time and space to make informed choices. Whether you are seeking a routine prenatal scan, a second opinion, or specialised fetal medicine consultation, Artemis Hospitals is equipped to support you at every step of your pregnancy journey.
Article by Dr. Kanika Singh
Sr. Consultant - Medical Genetics
Artemis Hospitals
