Combined first trimester screening for Down syndrome

Between the 11th and 14th week of pregnancy

For the timely detection of possible chromosomal abnormalities and malformations of the fetus, pregnant women are recommended to undergo a combined first trimester screening between the 11th and 14th week of pregnancy. The combined first trimester screening is the most effective method for determining the presence of Down syndrome and other chromosomal abnormalities; it detects approximately 90% of pregnancies with detected Down syndrome and chromosomal abnormalities.

Chromosomal abnormalities and fetal malformations are one of the main causes of perinatal mortality and morbidity. Although the vast majority of children are completely healthy, congenital anomalies occur in 3-5% of all newborns. One in a hundred children has a physical or mental handicap, with Down syndrome (an extra chromosome 21) being among the most common cause of this condition. Every woman carries the risk of giving birth to a child with Down syndrome or another chromosomal abnormality, but it is higher in older pregnant women.

Risk evulation

The combination of these 3 parameters is the safest way to assess the risk:

  • Mother's age
  • Ultrasound findings between the 11th and 14th week of pregnancy: nuchal fold thickness (measurement of fluid in the neck area of the fetus – every fetus has some liquid present, but in a fetus with Down syndrome, usually a larger quantity is found), nasal bone length, frontomaxillary facial (FMF) angle , Doppler ultrasound assessment of the ductus venosus flow, tricuspid regurgitation
  • Measurement of hormone values from the pregnant woman's blood between the 11th and 14th weeks of pregnancy (free β-hCG and PAPP-A: in pregnancies with Down's syndrome, there is a tendency for increased values of free β-hCG and decreased values of PAPP-A).

Invasive and non-invasive methods

The result of the combined screening only expresses the risk; such a test cannot confirm or deny the presence of Down syndrome with 100% accuracy. Genetic testing is the only certain method of detecting Down syndrome in a fetus. Non-invasive genetic methods using the mother's blood (NIPT tests) can rule out the existence of the most common chromosomal abnormalities (trisomies 21, 18, 13 and sex-linked chromosomal abnormalities) with more than 99% certainty.

Invasive genetic methods (amniocentesis and CVS) have been used for many years and have proven accurate in obtaining the entire karyotype of the fetus and are still considered the most accurate and complete genetic methods that can detect abnormalities in all 46 chromosomes.

The probability of chromosomal pathology, i.e. the degree of risk indicated by the combined screening, can help in deciding whether additional non-invasive (NIFTY test) or invasive diagnostic methods (CVS or amniocentesis) should be performed. The final decision on further diagnosis is exclusively personal, i.e. it is made by the parents.

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