Nuchal translucency and the bite test

The nuchal translucency test

The nuchal translucency test is done between the eleventh  and  thirteenth  week of pregnancy. It is an ultrasound examination (ultrasound), non-invasive, harmless to the unborn child, aimed at measuring the thickness of the nuchal subcutaneous tissue (nape) of the fetus, the one found between the skin and the spine.

This test is done to detect the existence of a possible Down syndrome or other hereditary chromosomal disorders affecting the fetus. In fact, an increased risk of these chromosomal pathologies corresponds to an excessive thickness of this area of ​​transfluence.

Just at this same stage of pregnancy, other (non-chromosomal) conditions can also be detected such as,

  • neural tube defects, the structure from which our central nervous system originates,
  • of abdominal wall defects,
  • limb abnormalities,
  • some congenital heart problems.

However, through the nuchal translucency test, it is possible to evaluate the risks that these pathologies may occur, but it is not possible to diagnose the conditions with precision.

However, if screening reveals a possible risk, this does not necessarily mean that there is an abnormality, it just implies that further tests will be needed( 1 ).

He bitest (o duo test)

The bitest (bi test, but also duo test) consists of a maternal blood sample, which is done at the time of the ultrasound relating to ducal transfluency, in order to make it more reliable.

This test consists in measuring the concentration in maternal blood of two placental substances,

  • the beta fraction of human Koranic gonadotropin, which is a glycoprotein with hormonal activity;
  • plasma protein associated with pregnancy (PAPP-A), which is a high molecular weight glycoprotein;

It is a test that is used to evaluate the dosage of the levels of the two hormones produced by the placenta.

The two combined tests – that of the nuchal translucency ultrasound, and the bitest blood sampling – correctly defined as the “first trimester combined test” – have the sensitivity, out of 100 cases, to detect the existence of 82 fetuses with chromosomal alterations, with 5% false positives.

According to the new LEA –  Essential Levels of Assistance –  the combined screening of the bite test and nuchal translucency are free for all pregnant women, regardless of age( 2 ).

The accuracy of the exams

When not accompanied by blood tests, this screening has a 75% accuracy in predicting Down syndrome. When combined with a blood test, its accuracy increases to 85%.

For women who obtain results indicating some possible risk, genetic tests are offered to be done through other procedures, such as amniocentesis or CVS.

How this test is done

The nuchal nuchal translucency test, as mentioned, is performed by means of an abdominal ultrasound which can be performed between 11 weeks and 4 days, and 13 weeks and 6 days of fetal age. To this end,

  • the woman will be asked to lie down on a bed,
  • a gel will be applied to your lower abdomen,
  • subsequently, a device called a transducer will be moved gently on the abdomen, with rotational movements,
  • the transducer transmits ultrasonic waves, which will be bounced back and – through these – the ultrasound machine will be able to reproduce the detected images on a screen.

Ultrasound is done to be able to see many things. Also including,

  • the size of the fetus,
  • heartbeat,
  • the general anatomy or structure of the fetus,
  • how many fetuses are present.

At this stage of pregnancy, the fetus is still too small for a detailed examination of its body structure to be possible: therefore this test will be done around the 18th week.

Nuchal translucency must be measured very accurately. However, sometimes it can be difficult to obtain any result, due to the position of the fetus at the time of the examination.

To make this measurement, it may sometimes be necessary to resort to a transvaginal ultrasound .

In transvaginal ultrasound, a transducer with a special shape is inserted into the vagina to give better images, and this is because in this case the device is closer to the fetus. If a transvaginal ultrasound is needed, the woman will be explained the procedure, and asked for her permission to follow it.

Test: side effects and risks

Doing an abdominal or transvaginal ultrasound has no side effects: therefore, immediately after the examination, the woman will be able to resume her normal activities.

Vaginal ultrasound.  As far as vaginal ultrasound is concerned, the existence of risks is not known, either for the fetus or for the mother: in fact, ultrasounds are nothing more than sound waves used to obtain images, and do not have radiation. Therefore, all the ultrasonographic examinations that are needed can be performed, without this entailing an increase in the risks due to repeated exposures.

Transvaginal ultrasound . Also with regard to transvaginal ultrasound, there are no risks for the fetus and for the mother, nor does it increase the probability of an abortion.

Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.

She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform,

Leave a Reply

Your email address will not be published. Required fields are marked *