Karyotype test on amniotic fluid
This is a complete study that includes an amniocentesis and genetic testing. The patient is seen by a geneticist who prepares their medical report, gives them a detailed explanation of the test that is going to be carried out, the advantages and the disadvantages of such and provides them with genetic counselling whenever necessary.
This test can also prevent any mother-fetus Rh blood type incompatibility problems.
The radiologist team carries out the amniocentesis and after the genetic testing when the whole procedure is complete; the patient is given the results. The related-miscarriage rate is very low (less than 0.5 %).
In addition to this, we always study the alpha-fetoprotein in the amniotic fluid to make sure there aren’t any neural tube closure defects.
The karyotype test is recommended in the following circumstances:
- Age: Women aged 35 and over
- Other children with chromosomal birth defects
- Chromosome abnormalities in parents
- Family history of a genetic disorder
- Increased risks of neural tube closure defects
- Altered screening
- Molecular diagnosis of monogenic diseases
- Family history of a disorder associated to the X for which there is no specific test
- Other pregnancies achieved with assisted reproductive techniques
Very recently the American Board of Medical Genetics published a document with advice on the prenatal screening for neural tube closure defects and aneuploidies stating that amniocentesis or chorionic villus sampling should still be made available for pregnant women aged 35 and over, as a method used to diagnose aneuploidy.
Chorionic villus sampling for fetal karyotyping
This is similar to the previous test if the material studied comes from a sample of the chorionic villus (placental tissue).
In this case, the karyotyping can be carried out a few weeks earlier than the amniocentesis (weeks 10-12 vs. 15-16), although the rate of related miscarriages after the sample has been obtained is higher (1%); only the extraembryonic material is examined, the AFP is not measured and the confined placental mosaicism can be a problem.
FISH in amniotic fluid
This is a study of the DNA using fluorescent probes (specific or complementary centromeric DNA sequences) and microscopy (Fluorescence In Situ Hybridization), to detect the existence of chromosomes without having to use cell cultures.
The big advantage of the test is that it is quick and results are obtained within 24-36 hours because it can be done without cell cultures, so the parents don’t have so long to wait and worry.
Normally the chromosomes 13, 18, 21, X and Y are studied because alterations to these cause 90% of the aneuploidies.
There are also specific probes that can detect certain mutations that are not captured on the resolution of conventional karyotyping but can be studied with FISH.
Prenatal study of monogenetic alterations
Many hereditary diseases are caused by mutations that occur in a specific gene and this can be passed down from one generation to another.
If the parents are affected by any of these diseases or is a carrier of a gene mutation, the molecular study on this mutation can be carried out on the cells taken from the amniotic fluid which is obtained by means of amniocentesis.