NT - Nuchal Translucency

Overview

NT is an echographic manifestation of the physiological accumulation of fluid in the fetal neck skin, in the first trimester of pregnancy.

NT increases in fetuses with chromosomal anomalies, heart defects, and other genetic syndromes.

Screening with NT detects 80% of fetuses with trisomy 21 and aneuploidy, with a false positive rate of up to 5%.

Combining NT with the double test (free bhCG in maternal serum and PAPP-A) increases the assessment up to 90%.

The certainty increases up to 95%, with a falsehood up to 3%, when the nasal bone, tricuspid flow, and flow in the venous duct are also evaluated.

NT Measurement Criteria

1. The gestational age for the echographic assessment of NT should be 11 weeks - 13 weeks and 6 days.
2. CRL should be between 45-84 mm.
3. The image must include the fetal thorax and head.
4. During the ultrasound, a sagittal view of the face is taken. The view includes the nose, anteriorly the palate, the translucent diencephalon in the center, and posteriorly the nuchal membrane.
5. The fetus should be in a neutral position, with its head in the same line as its back. When the fetal neck is in hyperextension, NT increases falsely whereas when the fetal neck is flexed, NT decreases falsely.
6. Care should be taken to differentiate between fetal skin and the amnion.
7. The assessment includes the widest part of the nuchal translucency.
8. The assessment is done by placing the arrow on the line and not on the fluid.
9. During the assessment, several measurements should be taken and an average measured value is taken.

Purpose of NT Assessment

In 5% of cases, around the neck, we may have the umbilical cord leading to a false increase in NT. In these cases, measurement above and below the umbilical cord leads to inaccurate NT measurement.

NT increases in fetuses with chromosomal diseases and fetal malformations.

a - Fetal malformations are a mistake in the phase of formation and development of fetal organs. They are assessed and identified through ultrasound.

b - Chromosomal diseases are an anomaly that affects the fetal chromosomes, which may not manifest with evident fetal malformations. The assessment includes invasive procedures such as: amniocentesis, chorionic villus sampling.

NT is assessed through transabdominal ultrasound except in cases such as: in a retroverted uterus, lack of a clear echogenic appearance where it is necessary to be assessed during a transvaginal ultrasound examination.

NT can be increased in many conditions such as aneuploidy (Down syndrome and other chromosomopathies), in syndromes and cardiac malformations. In these conditions, more detailed examinations such as chorionic villus sampling and amniocentesis are required.

Values and Parameters

Most normal fetuses have NT values ranging from 1.1-1.4 mm.

Most researchers take a maximum NT value of 2.5mm. There is no fixed limit value, however, the assessment is based on the CRL, NT value, and the patient's age based on algorithms based on the normal population. If NT values are above the defined limit then suspicions of pathology arise and at the same time, these parameters are also evaluated.

a - measurement of blood flow in the venous duct

b - presence or absence of the nasal bone

An increased NT value does not determine for sure, but only indicates a higher risk for fetal pathology. The certainty for a fetal pathology reaches 90% when three (three) parameters are assessed simultaneously, NT, flow in the venous duct, and assessment of the nasal bone in the first trimester of pregnancy.