The objectives of amniocentesis

What is amniocentesis performed for?

The word amniocentesis all it means is extraction of amniotic fluid. In the same way that when we draw blood from a vein in an arm we can do multiple studies in that blood, with the amniotic fluid we can do it too.

But the reality is that amniocentesis is mainly done to study the cells of the fetus that are "floating" in that liquid (coming from the skin, digestive system, urinary tract, ...) and determine their chromosomes. The best known study is the one that determines the fetal karyotype: after a culture of these cells - they are "seeded" as if they were seeds -, between 14 and 21 days, all the chromosomes are observed and their number is studied and if they present some anomaly. This is the time required to fully visualize all chromosomes.

A more "quick" study, although more limited, may be performed if clinical or personal circumstances so require. It can be done by two techniques: FISH (Fluorescence in situ hybridation) or by QF-PCR (Quantitative Fluorescence-Polymerase Chain Reaction).

Very briefly: this technique that studies, on the fetal cells of the amniotic fluid, is the number of certain chromosomes; does not study the entire chromosome, but only its number; for example we can know how many chromosomes 21 has a fetal cell: if you have three is a Down syndrome (usually has two).

This technique has the advantage that the result is very fast (24-48 hours), since it does not require the culture of the cells, but as a disadvantage it is that it does not detect all the chromosomal abnormalities (only detects the numerical of the studied chromosomes ), so it is also necessary to perform the classic culture (it takes 14-21 days to obtain the complete fetal karyotype).

In many centers already the two techniques are performed, since it is not necessary to extract more liquid to perform it and in a very short time we can have a result - which can be reassuring or not - although waiting for the definitive result of the study of all chromosomes in full form.

In addition, in the amniotic fluid we can study all kinds of infections, the presence or the production of certain substances by the fetus, etc.

Recently, a number of studies have emerged that may change some indications of amniocentesis:

That the karyotype is normal does not mean that the fetus can not have any disease of genetic origin. Diseases of genetic origin appear by alteration of some or some genes (thousands of them form the chromosomes) and we can not detect them unless they are studied specifically (theoretically they are hundreds or thousands). Although they are very rare in the general population, many carry important medical problems and a significant number of them are associated with mental retardation.

There are currently techniques ( CGH arrays ) that are able to determine in the amniotic fluid the genetic abnormalities of a group of serious diseases that would have a normal karyotype (the anomaly would be in one of the thousands of genes of a chromosome ). In addition, several hundred of them can be studied at the same time.

Realization or not of amniocentesis

In many medical centers in Spain, prenatal screening protocols have been put in place with the aim of reducing the number of amniocentesis; is based on the premise that amniocentesis would only be performed in those patients in whom the screening has given high risk, reducing the number of abortions related to the technique and, why not say, decrease health expenditure. Only amniocentesis would be done in the case of positive screening (considered positive from a certain level of risk), admitting a certain percentage of false negatives (negative screening when in fact the fetus does carry anomaly) to which it will not be performed amniocentesis and therefore will not be diagnosed before it is born.

Biochemical-combined screening

The best strategies are those based on combined screening, ie biochemistry (analysis of certain hormones in the blood of the mother) and ultrasound (determinations on the fetus observed by ultrasound), associated with the age of the mother.

Although encouraging data have been published in some specific care groups, the reality is that the generalization of these strategies has not been achieved, and in some cases, there are reasonable doubts about their implementation, especially in groups at risk for maternal age advanced (since it is not the same to detect a risk in a patient who does not have it a priori because it is young, to take a risk to an older woman who does it only because she is older).

We believe that the decision should be taken jointly with the pregnant woman after a good information about the technique, the actual risks of the same and the real risks of the patient to be carriers of a fetus with chromosomal abnormality.