10 questions about amniocentesis

Amniocentesis is not an insignificant examination, it is only offered to the mother-in-law in special circumstances. The explanations of Professor Cyril Huissoud, gynecologist-obstetrician.

Professor Cyril Huissoud is a gynecologist-obstetrician and coordinator of the multidisciplinary center for prenatal diagnosis (CPDPN) of the maternity ward of the Croix-Rousse hospital in Lyon

1. What is an amniocentesis?

  • This examination consists in taking about 20 ml of the amniotic fluid in which the fetus is bathed. The fetal cells contained in this sample are then isolated and cultured. Once they have multiplied, it is possible to establish the karyotype of the baby, that is to say, a representation of the 23 pairs of chromosomes present in each of its cells. From the karyotype study, the geneticist can identify any chromosomal abnormalities: missing chromosome, additional chromosome, chromosome rearrangement within the same chromosome.
  • Usually, amniocentesis is not done before 15 weeks of amenorrhea because before this term of pregnancy, it is not always possible to recover fetal cells in the amniotic fluid.
  • This examination is never imposed on parents, only proposed.

2. Is this examination advisable in case of suspicion of trisomy 21?

  • This is one of the great indications that can lead to amniocentesis. This is proposed from the outset when the ultrasound of the first trimester reveals an abnormal thickness of the nuchal translucency (greater than 3.5 mm). In fact, the thicker this region of the neck, the higher the associated risk of a chromosomal anomaly.
  • An amniocentesis is also proposed when the risk of giving birth to a baby with Down Syndrome equals or exceeds 1 in 50. This risk is evaluated by taking into account several parameters: the age of the mother (the older she is, the higher the risk increases), the determination of serum markers in the mother's blood and the measurement of the nuchal translucency of the fetus. If the risk is between 1/51 and 1/1000, before using amniocentesis, the mother can perform a test on free fetal DNA circulating in her blood (we are looking for an overrepresentation of the number of copies of chromosome 21). If this test performed from a simple blood test is positive, then an amniocentesis will be proposed because it alone can make the diagnosis of trisomy 21 from the karyotype. If this test is negative, surveillance will continue with the usual ultrasound examinations.

3. Can other anomalies visualized on ultrasound lead to amniocentesis?

  • Certain malformations of the organs (heart, brain, kidneys) are very frequently associated with chromosomal abnormalities. They therefore constitute an indication of amniocentesis.
  • Stunting of the fetus, especially if it is early and severe, may also be the manifestation of a chromosomal abnormality. But before performing an amniocentesis, it will be necessary to ensure by other examinations (in particular a Doppler echo) that this retardation of growth is not due to a malfunction of the arteries of the placenta.
  • If the ultrasound reveals a suspicion of spina bifida (a defect of closure of the neural tube), here also amniocentesis may be useful. It will allow on the one hand to look for a chromosomal anomaly sometimes associated with this malformation. And on the other hand to specify the diagnosis for better postnatal care: if we find in the amniotic fluid some molecules (especially acetylcholinesterase which is normally present in the cerebrospinal fluid of the fetus), it means that one is in the presence of a severe Spina Bifida, with an opening of the neural tube and thus a spinal cord which is not protected.
  • If on ultrasound a fetus has many edemas, this can be indicative of a metabolic disease - a dysfunction of the body affecting mainly lipids, carbohydrates or proteins. An amniocentesis will allow the diagnosis to be made by dosing certain metabolites in the amniotic fluid.

4. In case of infection contracted during pregnancy is amniocentesis useful?

  • If a pregnant woman contracts toxoplasmosis, amniocentesis can be performed to look for the parasite in the amniotic fluid. If it is present, it means that it has passed the placental barrier and the baby is infected. But that does not mean that he will suffer serious harm. In this context, the ultrasound surveillance of the fetus is reinforced, as well as the therapeutic management in order to limit as much as possible the risks related to the infection of the fetus by toxoplasma.
  • For other infections, such as cytomegalovirus infection, amniocentesis is not always relevant. On the one hand because there is no effective therapeutic known and on the other hand because the presence of virus in the liquid is not in itself a sign of gravity. On the other hand, ultrasound surveillance should be systematically strengthened to try to eliminate severe fetal damage.

5. Is amniocentesis used to screen for certain genetic diseases?

  • If there are heavy genetic diseases in the family (such as myopathy, cystic fibrosis, spinal muscular atrophy or certain forms of hemophilia), parents will be asked to perform amniocentesis to find out if their baby is he too reached.
  • When one of the parents carries a "balanced" chromosomal rearrangement (that is to say without pathological consequence), it can happen that it becomes unbalanced at the time of the formation of spermatozoa or eggs. The child can then inherit an "unbalanced" chromosomal rearrangement and source of illness or disability. Amniocentesis makes it possible to detect the possible transmission of this reshuffle, which has become dangerous.
  • In fact, there is no exhaustive list of family genetic risks that should lead to amniocentesis: this is decided on a case-by-case basis, in a dialogue between the geneticist of the prenatal diagnosis center and the parents.
  • In this context of the search for a genetic pathology, the examinations are more advanced than the establishment of the only karyotype because it is to study the genes of the fetus inside the chromosomes.

6. How does this exam take place?

  • Using a long needle and under ultrasound guidance to avoid touching the fetus, the doctor passes through the skin of the abdomen, fat, muscle, uterine wall and pocket of the uterus. waters. Arrived in the amniotic cavity, he can take a little liquid. If the placenta is in the anterior position (glued against the abdomen and not on the back side), it must also cross it.
  • This gesture is practiced without anesthesia - the pain is comparable to that of a puncture for a blood test - because it is difficult and painful to anesthetize the different "layers".

7. At what time of pregnancy does this test take place?

  • Most often between 15 and 20 weeks of amenorrhea.
  • But it can happen that it is done later, after the ultrasound of the 2nd trimester - between 20 and 23 weeks - because a morphological abnormality of the fetus was spotted at this stage whereas it was not yet visible in the 1st quarter.
  • Some malformations are detected even later, during the 3rd trimester. This is the case for certain bone diseases or brain malformations. They give rise to a late amniocentesis.

8. What are the risks of amniocentesis?

  • Rarely, the water pocket can break. In some cases, it is reconstituted and the amniotic fluid remains in sufficient quantity, allowing a smooth continuation of the pregnancy. In others, it does not recover and this can lead to serious complications, up to miscarriage. Very rarely, an infection can occur due to the intrusion of a germ.
  • This examination is most often performed in expert centers, by highly trained doctors. It is estimated that there is less than 0.5% risk of losing one's baby in the days following amniocentesis.
  • After amniocentesis, the mother is advised to rest for 24 to 48 hours and not to carry a heavy load.

9. How long to receive the results?

  • It takes between two to three weeks-depending on the rate of growth of the fetal cells-to establish the karyotype. In cases of complex genetic diseases, additional examinations and their interpretation may take several weeks.
  • Regarding trisomy 21, it is possible to have a result in 48 to 72 hours: we use a fast procedure (the FISH technique) which focuses on chromosomes 21, 18, 13 and sexual chromosomes X and Y.

10. What happens after the results?

  • The prenatal diagnosis center team presents parents with the prognosis for their baby, the risk of disability to which they are exposed.
  • It is always up to the parents, and especially the mother, to make the decision to continue the pregnancy or to interrupt it for medical reasons. This can not be imposed on them.
  • The prenatal diagnosis center can offer the help of a psychologist to accompany the parents in this difficult and painful decision.

© Isabelle Gravillon