Screening for aneuploidies: the combined test (bi-test)

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COMBINED SCREENING

The most effective method of screening for Down syndrome currently available is based on the combination of various factors. It starts from the maternal age to define the basic risk and this is then modified and customized for that specific pregnancy based on the results of an ultrasound scan performed between 11 and 13 +6 weeks and a sample of maternal blood. In this way it is possible to identify about 95% of fetuses with Down’s syndrome by performing an invasive examination in only 5% of pregnant women.

The ultrasound part of the screening consists in performing a series of measurements, such as fetal length (CRL), fetal heart rate, nuchal translucency thickness (NT), evaluation of the presence of nasal bone and evaluation of some functional abnormalities such as regurgitation (reflux) at the right atrioventricular heart valve (tricuspid) and retrograde flow at the level of a venous structure are in intrauterine life (ductus venosus).

The reasons for carrying out the screening test between 11 and 13 +6 weeks of pregnancy include the availability of an invasive examination such as villocentesis, which can be carried out at this time of pregnancy, which allows us to be certain of the presence or absence of a chromosomal anomaly, if the screening test indicates that the risk is high. Moreover, if the result of villocentesis is pathological and the couple decides to terminate the pregnancy, the procedure used for this purpose is safer and less traumatic for the woman at 11-14 weeks rather than after 16 weeks.

NUCHAL TRANSLUCENCY

NT is a liquid retronucal collection visible by ultrasound examination behind the fetal neck between the skin of the neck and the cervical-thoracic paravertebral tissues and is present in all fetuses between the 11th and 14th week of pregnancy.

In fetuses with trisomy 21 (Down’s syndrome) and other chromosomal abnormalities (e.g. trisomies 18 and 13) the thickness of the NT is in most cases increased compared to fetuses with normal chromosomal equipment.

MATERNAL BLOOD TEST

In pregnancies with Down’s syndrome or other chromosomal abnormalities, it is possible to find variations in the values of some substances produced by the placenta (whose cells have the same chromosome as the foetal cells) and which can be measured in maternal blood. In particular, Freeß-hCG and PAPP-A are the two substances used to assess the risk for chromosomal diseases in combination with the ultrasound examination described above.

RISK CALCULATION

The data from the ultrasound evaluation and the maternal blood test are included in a computer program that allows the combination of the various factors among themselves and with the maternal age and the calculation of the personal risk for Down’s syndrome and other chromosomal abnormalities in that particular pregnancy.

The risk calculation program is provided by the Fetal Medicine Foundation (FMF) in London to doctors who are accredited after theoretical and practical training and after an examination of competence and are reassessed annually on the basis of measurements made by them, thus ensuring the quality of execution of the test and the maintenance of the standards of execution of the test (the list of accredited operators worldwide is available at www.fetalmedicine.com). The operators of the PRENATAL DIAGNOSIS UNIT of the “VILLA MARGHERITA” nursing home meet these requirements and are regularly accredited, thus ensuring the optimal execution of the risk assessment.

SCREENING FOR PREECLAMPSIA (GESTOSIS) AND FETAL GROWTH RETARDATION

A recent development of ultrasound at 11-13 weeks +6 days was to be able to identify patients at increased risk of developing preeclampsia during pregnancy.

Preeclampsia is characterized by the appearance of high blood pressure and presence of protein in the urine in the second half of pregnancy.

Preeclampsia affects about 2% of pregnancies with serious risks for the mother and the fetus, especially in cases with early onset, before the 32nd week. The mechanism by which preeclampsia develops is based on insufficient placental development, which can be predicted by the combination of certain factors:

  • age, breed, weight, previous preeclampsia
  • measurement of maternal blood pressure
  • evaluation of blood flow in the uterine arteries supplying the uterus and the placenta
  • measurement of placental factors in maternal blood (PAPP-A, PlGF)

The combination of these factors allows to diagnose 90% of the patients who will develop preeclampsia, thus being able to apply a proper management of cases at risk and the management of appropriate therapy.

They are performed by appointment:

OSTETRIC ECOGRAPHY 1st and 2nd LEVEL

ANEUPLOIDY SCREENING: COMBINED TEST (BI-TEST)

PREECLAMPSIA SCREENING AND GROWTH RETARDATION

HARMONY TEST

GENETIC OR PREMORPHOLOGICAL ULTRASONOGRAPHY

FETAL ECHOCARDIOGRAPHY

STUDY OF PREGNANCIES AND BLOOD CLOTS

CARDIOTOCOGRAPHY

AMANIOCEANTICS

VILLOCENTESI