NT thickness has come 0.9m in 13th week pregnancy. is this worrying situation

Patient

Q: My wife if 13th week pregnant and recently we did ultrasound. The thickness of NT has come 0.9mm and doctor has said that the baby has congenital abnormalities. Is this a worrying situation for us?

Doctor

A:   I believe there is an error in the question and the measured value is 9mm or 0.9 cm as 0.9 mm is rarely an accurate value at 13 weeks. In any case, it is the increased nuchal translucency that is suggestive of an abnormal baby so I shall answer the question accordingly.
NT scan is a test performed between 11 - 14 weeks of gestation. At 12 weeks of gestational age, an "average" nuchal thickness of 2.5 mm is usually observed; this is generally taken as the cutoff value and NT measuring greater than 2.5 - 3mm are reported as abnormal.
Increased nuchal translucency on a NT scan is defined as an abnormal collection of fluid under the skin at the back of the fetus' neck. Abnormal nuchal translucency is an ultrasound "marker", which indicates increased risk for certain problems such as chromosomal abnormalities (trisomies) like Down's syndrome etc.
Between 65 to 85% of trisomic fetuses will have a large nuchal thickness. Further, other, non-trisomic abnormalities may also demonstrate an enlarged nuchal transparency. This leaves the measurement of nuchal transparency as a potentially useful 1st trimester screening tool.
These results are subject to interpersonal diagnostic errors as up to 13% of chromosomally normal fetuses present with a nuchal lucency of greater than 2.5mm.
For greater accuracy of predicting risks, the outcome of the nuchal scan should be combined with the results of simultaneous maternal blood tests such as double marker test in the first trimester. In pregnancies affected by Down's syndrome there is a tendency for the levels of human chorionic gonadotropin (hCG) to be increased and pregnancy-associated plasma protein A (PAPP-A) to be decreased. In your case I would recommend that you go in for the second line diagnostic tests such as a congenital anomaly scan at 16 combined with a triple marker blood test. In case of a strong positive risk factor in your case or if the second trimester screenings are also indicative of a high risk, the options would be to go in for more invasive tests such as amniocentesis that tests the chromosomes of the child and are much more accurate indicators of an abnormal child.
Discuss the case with your treating obstetrician and take an informed decision based on the same.

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