Today, amniocentesis is still the technique of reference in the genetic study of the fetus before birth. It consists of obtaining a small volume of the amniotic fluid that bathes the fetus during pregnancy by transabdominal puncture. Floating in these fluids may be fetal cells whose genetic load can be studied. The main indication for amniocentesis today is the study of aneuploidies, genetic defects that are usually associated with the mother's age (although they can also occur in young women).
Amniocentesis is an invasive technique that can cause rupture of the amniotic sac or miscarriage in a small percentage of cases. In order to avoid this risk, which is individually small, but population-wise important (i.e., statistically, in a hospital that performs many amniocentesis, the occurrence of some miscarriages or rupture of membranes is statistically expected, a pathology that depends directly and inevitably on a medical act), different options have been developed to try to avoid this test, and its sister technique, chorionic villus sampling.
These techniques can be grouped as follows:
- Ultrasound techniques
- : The measurement of certain fetal parameters (NCD, nuchal fold, nuchal translucency, femur length) or the identification of certain characteristics (presence of nasal bone, absence of malformations) are useful in the identification of possible babies with aneuploidies.
- Analytical techniques
- : Certain substances present in maternal blood during pregnancy can be altered if the pregnancy is aneuploid (PAPPA, B-HCG, Estradiol...).
- Statistical techniques
- The occurrence of familial cases of aneuploidy, or maternal age are statistically associated with the presence of aneuploidy in pregnancy.
- Mixed techniques
- >The combination of all the above techniques can increase the detection of aneuploid fetuses. Nowadays, together with specialized ultrasound, it is the most widely used technique in obstetrics, and in general it is offered to all pregnant women in Spain. The usual protocol is to perform a blood test around the 10-12th week in order to obtain the analytical parameters and a small survey and ultrasound around the 12-14th week in which the statistical and ultrasound parameters are obtained. These parameters are then integrated on the basis of a computerized algorithm, which quantifies the risk of the pregnant woman and compares it with what is desirable and with the group of women of her age.
- Genetic techniques
- The evolution of non-invasive techniques has generated some very interesting genetic-based tests, which are much more reliable than the previous ones (although amniocentesis is still better). These techniques are based on fetal DNA circulating in maternal blood.
During pregnancy the fetus grows continuously and remodels its organs and tissues. This means that some of the fetal cells die irremediably, to be replaced by new ones. Since the fetus has no contact with the outside world, the fetal detritus is expelled into the mother's bloodstream, which will take care of its subsequent metabolization and/or elimination through her liver and kidneys. Thus, as part of these cellular detritus that can be found in maternal blood, we can find fragments of fetal DNA. After sequencing the DNA present in maternal blood, we can identify the presence of DNA imbalances that can be explained by fetal aneuploidy. Nowadays these tests are highly recommended and reliable, although they have not yet become widespread because they have not yet been included universally in the public health system. In general, these tests must be financed directly by the couples, and their cost is currently 400-600 euros.
On the other hand, although these tests are purely genetic, for technical reasons they cannot give absolute diagnoses, so, like the previous tests (ultrasound, analytical and mixed), in the event that they determine a probable genetic alteration of the embryo (which occurs infrequently) the test must be verified by amniocentesis.
Thus, all the tests mentioned above only replace amniocentesis in the case of a negative test in low-risk pregnancies. If there is a possibility, or in any case if a high risk of genetic alteration has been determined, amniocentesis should always be recommended, as it is still the test that most reliably determines the presence of a genetic alteration of the aneuploidy type (and others) in fetuses.
It is therefore concluded, as is logical, that the opinion of the specialist should be followed by this and other questions raised in pregnancy.
