If NT Scan Is Not Normal

The Nuchal Translucency scan gives us the values of nuchal fold thickness, which is the subcutaneous space behind the baby’s neck. The normal NT value ranges from 1.6-2.4 mm in measurement. Any value above the normal limit is considered abnormal, suggesting a risk of fetal chromosomal abnormalities.

How common are abnormal NT scans?

Abnormal NT scans are mostly sporadic abnormalities, which means the abnormality has occurred through mutation and not an inheritance. An estimated 40% of fetuses with diaphragmatic hernia have increased NT thickness. 

Increased NT is also seen in both 85% of chromosomally abnormal and 40% of chromosomally normal fetuses with exomphalos (a defect in the abdominal wall of the baby).

NT thickness

Why is the NT scan abnormal?

There are a few risk factors that facilitate abnormal NT scans. Increased age of the pregnant woman(>35) might be a predisposing factor. Mostly the abnormality is due to fetal chromosomal abnormalities such as Down’s syndrome or Turner’s syndrome or Edwards syndrome, cardiovascular failure, and diaphragmatic hernia.

What is the high risk in an NT scan?

The risk rate of the NT scan is classified as low, intermediate, and high risk for precise evaluation by the obstetrician. The high risk would be an NT of 6mm, which shows a high probability of Down’s syndrome.

 Yet, there can be the minimal possibility that it isn’t. The appearance of the nasal bone helps in detecting trisomy 21 with greater accuracy. Hence, the absence of it also adds up to the increased risk of the NT scan.

What causes high NT?

A high NT is usually a result of the rise in the nuchal translucency fluid. A high NT doesn’t always indicate a chromosomal abnormality. It is said that an NT scan has five percent false positives. Hydrops or lymphatic obstruction is the suggested cause of increased nuchal thickness. 

Especially in the third trimester, an abnormally thickened nuchal fold may go away, but the risk of karyotypic abnormalities is unaffected. Parvovirus infection should be taken into account as a differential diagnosis if the NT is higher.

What causes high NT

Can a baby with Down syndrome have a normal nuchal translucency?

An NT value less than 2.9mm is unlikely to have Down’s syndrome. An increase in the NT increases the risk of Down’s syndrome. Also, NT doesn’t provide us with exact results every time. 

A normal NT might also rarely develop into a chromosomal abnormality, said that babies with Down’s syndrome have elevated hCG (Human Chorionic Gonadotropin) levels. Concerning the intensity of risk, your obstetrician will indulge in an alternative method, one such is amniocentesis.

down syndrome baby

What is the normal range of NT at 13 weeks?

A nuchal Translucency scan is done between 11 and 14 weeks of gestation. The measurement of nuchal translucency rises with gestational age which is the period from conception to delivery of the baby.
The normal limits fall from 1.6 mm-2.4 mm. However, at the period of 13 weeks of gestation, the NT might reach 2.8mm-3mm. These values are considered to be at a lower level of risk.
What is the normal range of NT at 13 weeks

What to do if the NT scan is not normal?

The first thing to do is to try to stay calm and take a deep breath. It might probably upset the mother, and the chances are higher to get anxious. Your obstetrician will provide you with a better follow-up, and further tests to evaluate the baby in the best possible ways.  It is to be noted that an NT scan is not a diagnostic test. A combined test of NT scan and blood markers seems to give 85-90% accurate results. Chorionic villus sampling and amniocentesis are also further tests to detect the risk of abnormality in the baby.
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