It is a screening test in form of ultrasound scan done from 11 to 14 weeks of pregnancy. To be precise it is done from 11 weeks 3 days to 13 weeks 6 days of pregnancy. It is usually done along with a blood test called dual marker and these both form a combined screening test.
It is known as NT /NB scan as doctor measures the nuchal anslucency (NT) and nasal bone (NB) of the fetus. It is also commonly known by name of level-I scan. The radiologist measures a fluid-filled space behind your baby’s neck known as nuchal transluceny. This measurement can help to detect the risk of your baby having a chromosomal abnormality such as Down syndrome.
In addition to nuchal translucency the doctor looks for nasal bone of fetus. Absent or short nasal bone indicates the possibility of underlying chromosomal abnormalities.
Other things like developing limbs, stomach bubble, intracranial structures, developing spine , placenta, cervical length etc are also assessed in the same scan. Some structural abnormalities can be ruled out in this scan like absent limbs, skeletal abnormalities , any abdominal wall defect with protrusion of the abdominal contents out etc. So it’s a type of early scan to rule out some abnormalities which are lethal and there is no point in carrying the pregnancy further.
In addition to these now a days pre-eclamsia screening is routinely done with the NT/NB scan. It helps to assess the risk of patient of developing hypertension ( increased blood pressure) in the course of pregnancy. In case of high risk the obstetrician gives some prophylactic drugs to the patient. Pre-eclampsia is a very common cause of bad obstetrics outcone like intrauterine fetal demise, growth restriction, premature deliveries etc.
In case this is your first scan it gives the additional information about the site of pregnancy and the exact dating.
So all in all it’s a very useful non invasive test and every patient must be recommended this scan to rule out underlying problems fetus may have.It also gives reassurance to the mother that everything inside is going fine and relieve her of undue stress.
What are the indications and right time to get NT /NB scan?
Now a days with the advancement of the medical field it is a routine to get level I USG for all the expecting mothers whether there is any history of any congenital abnormality in previous issues or not.
The importance increases if the maternal age is high, previous history of birth defects , previous fetal demise, maternal high BP and diabetes.
A nuchal translucency scan is done between 11 and 14 weeks of pregnancy. Ideally the time is 11weeks 3 days to 13 weeks 6 days.
What to expect and how to prepare yourself while going for nuchal translucency scan.
The first thing is you need not be fasting before going for scan. Many patients starve themselves before these scan and end up feeling miserable. There is no role of fasting in the obstetric scan.
You need to drink some water and you must have moderately full urinary bladder. There is no need to have extra-full bladder, just moderate pressure will be good for the scan.
Normally the scan is done trans- abdominally. The radiologist will apply some gel and gently move the ultrasound probe across your abdomen. Usually it takes about 15-20 minutes for the scan. However it all depends on the position of your baby. If the position is favorable the scan can be done easily in one go. Many a times you will have to wait until the baby lies in the favorable position required by the radiologist. You may be asked to come repeatedly in the ultrasound room after waiting for some time.
Some times trans vaginal ultrasound needs to be done for better visualization of the fetus. In that case ultrasound probe is gently inserted into your vagina which may feel a bit uncomfortable but usually is not painful.
What does the results tell?
The NT/NB scan results are usually combined with a blood test ( Dual markers) and these both assess the combined risk of chromosomal disorders in pregnancy.
Remember that a normal screening result (screen negative) isn’t a guarantee that your baby has normal chromosomes, but it does suggest that a problem is unlikely. Likewise, an abnormal screening result (screen positive) doesn’t mean that your baby has a chromosomal problem – just that your baby is more likely to have one. Most screen-positive babies turn out not to have a problem: Only about a third of babies with an increased nuchal translucency measurement turn out to have chromosomal defects.
Results are usually reported as ‘low risk’ or ‘high risk’. If the result comes out to be low risk then the pregnancy is considered to be normal and no further tests are usually done. If the result comes to be moderate or high risk then further assessment is done by invasive procedures such as chorionic venous sampling and amniocentesis which will give you a definitive result. A non invasive blood test know as NIPT (Non invasive prenatal testing) is also available now a days to diagnose the chromosomal abnormalities. Your obstetrician or genetic counselor will help you to choose between these tests.