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NT NB Scan During Pregnancy: What is It, When is It Done, What Will You Find Out from the Scan?

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While pregnancy is a beautiful journey of enjoying the glimpses of your baby’s heartbeats and experiencing the kicks and jerks of your baby. Having said that, pregnancy also brings a series of appointments and screenings essential for your little one. 

Screenings are required to understand the development and growth of the baby and check the risks of chromosomal abnormalities in the fetus. Nuchal translucency (NT), Nasal Bone (NB) scan is one such screening done to predict the possibilities of chromosomal anomalies like Down syndrome, Patau’s syndrome, or Edwards syndrome.

This article speaks in detail about the Nuchal translucency (NT), Nasal Bone (NB) scan, its purpose, its working, and the findings of the scan.   

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 Nuchal translucency (NT), Nasal Bone (NB) Scan during pregnancy: What is it?

 Nuchal translucency (NT), Nasal Bone (NB) scan is a screening test commonly done in the first trimester between week 11 to week 13 of pregnancy. The test measures the amount of clear tissue present at the back of the neck of the fetus. This is the NT scan or Nuchal Translucency Scan

Usually, not many clear tissues or fluid are present at the back of the fetal neck. But too much clear fluid can indicate chromosomal anomalies.

NB scan or Nasal Bone Scan is done along with an NT scan to check the baby’s nasal bone where the absence of nasal bone can be a red flag for Down syndrome. 

Nuchal_edema_in_Down_Syndrome_Dr._W._Moroder
Nuchal edema in Down Syndrome

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The need for NT NB Scan

The Nuchal translucency (NT), Nasal Bone (NB) scan is a predictive test and not a diagnostic test. This means NT NB can predict the possibility of an abnormality but cannot confirm the same. 

The clear tissues at the back of the fetal neck disappear after week 15 of pregnancy. Hence, NT NB scans are ideally done at the end of the first trimester. 

The human cell consists of 23 pairs of chromosomes, including a pair of sex chromosomes- XX in females and XY in males. The addition or deletion of chromosomes or a segment of chromosomes can cause chromosomal abnormalities. This scan predicts one of the below chromosomal abnormalities in the baby.

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  • Down syndrome or Trisomy 21, where the baby will have an extra copy of chromosome 21. 
  • Patau’s syndrome or Trisomy 13- the presence of an extra copy of chromosome 13.
  • Edwards syndrome or Trisomy 18- Individuals are born with an extra chromosome 18.

These conditions can cause a delay in development and unusual physical characteristics such as,

  • Stunted growth
  • Eyes with upward slant or impairment
  • Weak bones and muscles
  • Low weight at birth
  • High risk of other health problems

The NT NB scans can help identify whether the baby is likely to develop these conditions or not. 

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How is the NT ND Scan performed?

NT NB tests are usually conjugated with routine blood serum marker tests or dual marker tests to eliminate the presence of any chromosomal abnormality in the fetus. 

The NT NB tests are a specialized routine abdominal ultrasound scan. You can alternatively have a transvaginal ultrasound. The ultrasound pictures are clearer when the bladder is full. And so, it is advised to drink water an hour before the test.

The images obtained through the ultrasound scan allow the doctor to measure the NT levels and note the presence of the nasal bone. The age and probable due date are also mentioned in the report while calculating the baby’s risk of having an abnormality.  

NT NB Scan findings

The results of the NT NB scans are generally available on the same day of the test. It is important to note that these tests are not diagnostic nor conclude any abnormality. But, these can guide you if you require a more conclusive diagnostic test such as NIPT, chorionic villus sampling (CVS), or amniocentesis. 

NIPT: A maternal blood test done as early as week 8 of pregnancy checks the cell-free fetal DNA present in the maternal blood. 

Chorionic Villus Sampling (CVS): An ultrasound-guided test is done, where a segment of the placental tissue is extracted to evaluate the fetus’s developmental status. This test is usually performed between week 11 to week 14 of pregnancy. 

Amniocentesis: Usually done between week 15 to week 18 of pregnancy. Guided by an ultrasound, a certain volume of amniotic fluid is drawn to diagnose chromosomal disparities in the fetus during amniocentesis. 

However, CVS and amniocentesis carry pregnancy risks and are usually preferable after a positive NT NB test result. 

NT scan combined with the blood tests is about 85% accurate in predicting the chromosomal abnormality in the fetus. While stand-alone, NT NB tests are only 75% accurate in predictions. 

Also, NT NB tests have a false positive rate of 5%, i.e., 5% of the women testing positive for the test have normal children, devoid of chromosomal abnormalities. Hence, NIPT, CVS, or amniocentesis is suggested after a positive NT NB result. 

Conclusion

NT NB scans are regular ultrasound scans done in the first trimester where the NT scan measures the clear tissues at the back of the fetal neck region. And an NB scan checks the presence of the nasal bone of the fetus. NT NB scan predicts if the baby has chromosomal abnormalities like trisomy 21, trisomy 18, and trisomy 13 that can cause developmental deformities in the child. 

A positive NT NB scan means you need to go for a definitive diagnostic test such as NIPT, CVS, or amniocentesis. To know more about these screening and diagnostic tests or get them done, check with your doctor at the earliest.

Pooja N Swamy (M.Sc Biomedical Sciences)
Pooja Holds Master's Degree in Biomedical Sciences from Vellore Institute of Technology. She believes in strong research and clear understanding over the subject. Always determined and focussed with a thirst to learn new things. Hardworking, flexible and grounded to align with the goals.

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