PRENATAL

Reasons for prenatal chromosomal diagnosis:

  • Increased maternal age (35 years old or greater).
  • Positive maternal serum screening result indicating an increased risk of a chromosomally abnormal fetus.
  • Previous stillbirth with a potentially viable chromosome abnormality.
  • Previous birth of a child with multiple anomalies.
  • Parental chromosome rearrangement, chromosome mosaicism, or sex chromosome aneuploidy.
  • Repeated miscarriages.
  • Abnormal ultrasound findings.
  • Intracytoplasmic sperm injection, or other medical intervention that increases the probability of   chromosome abnormalities.

If any of these indications are present, it is advisable to seek genetic counselling. Previous findings, further prenatal diagnostic options, and indication for these can be individually explained for the couple.  

On the basis of this information, a personal decision can then be taken about whether or not to proceed with further diagnostic methods.


Techniques used to collect fetal cells for genetic testing:

  • CVS – chorionic villous sampling

Performed between 10-12 weeks of gestation. Under ultrasound guidance, a catheter is passed through the cervix or through the abdominal wall into the uterus, and a sample of 15/20-mg chorionic villi is obtained. Chromosome analysis is carried out to determine the karyotype of the fetus using short-term and long-term cultures. The risk of miscarriage after CVS is reported to be 1-2%.

  •  Amniocentesis

Performed between 15-20 weeks of pregnancy. Under ultrasound guidance, a needle is passed through the abdominal wall into the amniotic cavity inside the uterus, 10/20-ml of amniotic fluid are collected and the fetal cells are grown in culture for chromosomal analyses; the fluid can also be used for the measurement of substances, AFAP, ACHE, hormones, and enzymes. The procedure includes risks for miscarriage (0.5-1.0%), and maternal Rh sensitization.

  • Cordocentesis

Performed from the 20th weeks of gestation. 2-ml sample of fetal blood is collected by puncturing the fetus umbilical cord. The sample is suitable for obtaining chromosomal analysis results and looking for certain blood disorders with high levels of accuracy. The procedure includes risks for miscarriage (1-2%), premature rupture of membranes, and blood loss from the puncture site.

  • Deviations from the normal chromosomal number or structure are detectable in 5-10% of all conceptions and in 0,5% of live born babies, some being recognised as known syndromes. The most commonly detectable prenatal aneuploidies include trisomy 13, trisomy 18, and trisomy 21.                                                                                      
  • FISH test results are used as an aid in the diagnosis of numerical abnormalities of chromosomes 13, 18, 21, X and Y, in conjunction with other information currently used in prenatal diagnosis. A multicolour DNA Probe kit - Aneuvysion - is used as in-vitro diagnostic test for detection of these abnormalities via FISH, on interphase nuclei and metaphase cells obtained from amniotic fluid samples. FISH technique also allows geneticists to identify prenatal or postnatal small chromosomal rearrangements such as deletions, duplications, or translocations.

 

Metaphase FISH limits: 

  • provides numerical and structural information only about selected chromosomes;
  • consults the lab FISH probes list prior to order a specific test;
  • amniotic fluid samples contaminated with blood may be excluded.


INTERPHASE FISH for PRENATAL DIAGNOSIS:

  • Abnormal ultrasound indicating a probable trisomy 13, 18, 21 or Turner Syndrome.
  • Late gestational age (20 weeks or greater) with a positive maternal serum screening for Down Syndrome   or trisomy 18. 
  • Late gestational age (20 weeks or greater) with a late maternal age (40 years old or greater), when the patient did not have a maternal serum screen.


Interphase FISH limitations:

  • provides only numerical information about selected chromosomes: 13, 18, 21, X and Y;
  • does not detect structural rearrangements, mosaicism, or other numerical abnormalities;
  • amniotic fluid samples contaminated with blood may be excluded
  • amniotic fluid samples from early gestation pregnancies may not contain enough cells for informative results in the FISH analysis;
  • amniotic fluid samples from late gestation pregnancies may contain excess debris, which could make the FISH assay uninformative.


Counseling and informed consent are recommended for genetic testing.

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