Unidad de Reproducción de la Clínica Vistahermosa

Fetal karyotype in amniotic fluid

We perform a joint study including amniocentesis and genetic analysis. The patient is seen by the geneticist who takes a clinical history, makes a detailed explanation of the test with its advantages and disadvantages and, where necessary, gives genetic counselling.

In this way, problems due to blood-type incompatibility between mother and foetus are prevented.

Later, amniocentesis is performed by the radiology team, and after genetic examination, the patient receives the results at the end of the process. The rate of miscarriage is low (somewhat less than 0.5%).

We always investigate the alpha-fetoprotein in the amniotic fluid to detect defects associated with interference in the closure of the neural tube.

Karyotype study is indicated in the following circumstances:

• age: woman of 35 or older
• previous child with chromosome defects
• chromosome abnormalities in any of the progenitors
• family background of genetic defects
• increased risk of defects in neural tube closure
• altered screening
• molecular diagnosis of monogenetic disease
• family background of disturbances linked to the X for which there is no specific test
• pregnancies obtained by assisted reproduction techniques

The American Association of Medical Genetics has recently published new advice on prenatal screening of neural tube defects and aneuploidy, recommending that amniocentesis or chorionic villi sampling should be offered to pregnant women of 35 and over as the method of choice for the diagnosis of aneuploidy.

Fetal karyotype in chorionic villi

This is a study similar to the one above where the material investigated comes from a biopsy of the chorionic villi region.

It enables the determination of the karyotype some weeks earlier than the study of amniotic fluid (weeks 10-12 compared with 15-16), although the rate of miscarriage after extraction of material is higher (1%). Only extra-embryonic material is examined, AFP are not measured and mosaicism restricted to the placenta can be a problem.

FISH in amniotic fluid

This is a study by means of DNA probes (DNA complementary to specific sequences) and microscopic observation of fluorescence (Fluorescence in situ hybridisation) of chromosomes without the need for cellular cultivation.

The advantage of this test is in its speed, with results being obtained in 24-36h and in not needing to cultivate cells, which reduces the parents' anxiety.

Chromosomes 13, 18, 21, X and Y are routinely studied where abnormalities cause more than 90% of the aneuploidies.

There are also specific probes for some mutations that fall below the resolution of conventional karyotype technique and that can be studied with FISH.

Prenatal study of monogenetic abnormalities

Many hereditary diseases are due to mutations in a specific gene that can be transmitted to descendents.

If any progenitor is affected by any of these diseases or is a carrier of a mutation, a molecular study for this mutation can be performed in cells from the amniotic fluid obtained by amniocentesis.