The objectives of an amniocentesis

What is amniocentesis performed for?

The word amniocentesis simply means the removal of amniotic fluid. In the same way that when we draw blood from a vein in an arm we can do multiple studies on that blood, we can do the same with amniotic fluid.

But the reality is that amniocentesis is mainly performed to study the cells of the fetus that are “floating” in the fluid (from the skin, digestive tract, urinary tract, etc.) and to determine their chromosomes. The best known study is the one that determines the fetal karyotype: after a culture of these cells – they are “sown” as if they were seeds -, between 14 and 21 days, all the chromosomes are observed and their number is studied and whether they present any anomaly. This is the time needed to completely visualize all the chromosomes.

A more “rapid” study can also be performed, although more limited, if clinical or personal circumstances so require. It can be performed using two techniques: FISH (Fluorescence in situ hybridization) or QF-PCR (Quantitative Fluorescence-Polymerase Chain Reaction).

To summarize: this technique studies the number of certain chromosomes in the fetal cells of the amniotic fluid; it does not study the entire chromosome, but only its number; for example, we can find out how many chromosomes 21 a fetal cell has: if it has three, it is Down syndrome (normally it has two).

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

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

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

Recently, another series of research has emerged that may change some of the indications for amniocentesis:

The fact that the karyotype is normal does not mean that the fetus cannot have a disease of genetic origin. Diseases of genetic origin appear due to alterations in one or more genes (thousands of them form the chromosomes) and we cannot detect them unless they are specifically studied (theoretically there are hundreds or thousands). Although they are very rare in the general population, many of them cause important medical problems and a significant number of them are associated with mental retardation.

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There are now techniques (called CGH arrays) that are capable of determining, in amniotic fluid, the genetic anomalies 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.

To perform or not to perform amniocentesis

In many medical centers in Spain, prenatal screening protocols have been implemented with the idea of reducing the number of amniocenteses; this is based on the premise that amniocentesis would only be performed on those patients in whom the screening has shown a high risk, thus reducing the number of abortions related to the technique and, why not say it, reducing health care costs. Amniocentesis would only be performed in the case of positive screening (it is considered positive above a certain level of risk), admitting a certain percentage of false negatives (negative screening when in fact the fetus does carry the anomaly) for which amniocentesis would not be performed and therefore would not be diagnosed before the fetus is born.

Biochemical-combined screening

The best strategies are those based on combined screening, i.e. biochemical (analysis of certain hormones in the mother’s blood) and ultrasound (certain measurements in the fetus observed by ultrasound), associated with the mother’s age.

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

We believe that the decision should be made jointly with the pregnant woman after good information about the technique, the real risks of the technique and the real risks for the patient of carrying a fetus with a chromosomal abnormality.