The trophoblast biopsy is a prenatal diagnostic examination consisting of removing a fragment of the trophoblast, the future placenta, in order to establish the karyotype of the fetus and to detect certain chromosomal abnormalities or genetic diseases. In some situations, it is a great alternative to amniocentesis.

The trophoblast is a structure in early pregnancy that will become, at the end of the first trimester, the placenta. These are the villi surrounding the egg. Produced by the division of the cells of the egg, they contain the same genetic material as the unborn baby.

The trophoblast biopsy (also called choriocentesis or chorionic villus sampling) consists of taking a very small fragment of this tissue, then analyzing it in the laboratory in order to establish the karyotype, a kind of “map” of the chromosomes of the future baby and to look for certain genetic abnormalities.

The trophoblast biopsy is not part of the standard pregnancy follow-up. This is a prenatal diagnostic test only carried out in the event of suspected serious chromosomal malformations, in particular in the presence of a history of genetic disease (cystic fibrosis, myopathy, sickle cell disease, hereditary metabolic diseases) or following the results of combined screening. of trisomy 21 suggesting an increased risk. Over 200 genetic abnormalities can be diagnosed by cytogenetic, enzymatic or DNA studies on chorionic villi. The trophoblast biopsy can also be used to determine the sex of the baby at an early stage.

In accordance with the legislative framework governing prenatal diagnosis, the trophoblast biopsy is not compulsory: it is offered to the couple during a medical consultation, but they are free to perform it or not. The woman must also sign a consent before the examination (1).

The trophoblast biopsy is usually done between 11 and 14 weeks gestation. It therefore has the advantage of being able to be performed earlier during pregnancy than amniocentesis (examination consisting of taking amniotic fluid in order to establish the karyotype of the fetus) performed between 15 and 17 weeks. This makes it possible to reassure patients earlier and to minimize obstetric complications and the psychological impact of a possible medical termination of pregnancy (IMG) decided following the results of the examination.

The mother-to-be must present her blood group card. If she is rh negative, she should indeed receive an intravenous injection of anti-D immunoglobulins in order to avoid foeto-maternal incompatibility.

The examination takes place in outpatient surgery, in an ultrasound room, under strict aseptic conditions. It does not need to be fasting. An ultrasound is performed upstream in order to check whether all the conditions are met for the proper conduct of the examination, to precisely locate the trophoblast and to choose the most suitable method. There are indeed two techniques:

• through the abdominal wall: under ultrasound control, the practitioner inserts a long, thin sterile needle through the abdominal wall, then punctures a small fragment of the trophoblast. Local anesthesia may be performed at the injection site. The puncture itself is generally not painful but unpleasant, and can cause a contraction at the time of the puncture;
• vaginally: as during a gynecological examination, the gynecologist introduces a small forceps through the cervix and punctures a small fragment of trophoblast. This method carries a risk of infection and miscarriage is a little higher than with the abdominal method. This technique is therefore gradually abandoned by practitioners.

Sometimes the location of the trophoblast does not allow a biopsy to be taken. The examination will then be postponed or replaced by an amniocentesis a few weeks later.

The mother-to-be can quickly return home after the exam. She does not have to stay in bed, but it is best to stay at rest.

Even carried out under the best conditions, performing a trophoblast biopsy carries an estimated risk of miscarriage estimated at around 1% (2), compared with 0,5 to 1% for amniocentesis (3). Miscarriage can be caused by cracking membranes or infection. The risk is highest in the 8 to 10 days following the examination. In the event of bleeding, loss of fluid, fevers, abdominal pain, the expectant mother must therefore consult urgently.

The information sought by the trophoblast biopsy is specified during a consultation with a cytogenetic team and the obstetric team. The results are obtained in two stages:

• the number of chromosomes is obtained quickly (2 to 5 days) by a direct technique without culture;
• the complete karyotype requires cultivation, the time taken to obtain results is then of the same order as that of an amniocentesis, ie 10 to 15 days.