Villocentesis

Chorionic villus sampling is a major advance in prenatal diagnosis and has become a routine intervention for genetic risk pregnancy at our center. Chorionic villus sampling allows an accurate analysis of the fetal chromosomes as early as 11-12 weeks of amenorrhea versus the 15-20 weeks required to perform the amniocentesis.

Anatomical basis

The villi constitute a placental layer interposed between the basal decidua that adheres to the uterine wall, and the chorionic plate placed externally on the fetal side of the placenta. In the villi there are many cells in active reproduction that allow to study the karyotype.

Usefulness of the exam

ANALYSIS THAT CAN BE PERFORMED ON VILLAS

The analyzes that can be performed on the chorionic villus sample are:

• determination of the fetal chromosomal set (karyotype) to exclude the presence of any chromosomal anomalies (the indications for carrying out the examination in this case are the same already listed for the amniocentesis to which we refer for completeness) with the advantage of being able to obtain results in a very short time (with direct preparation 40-72 hours);
• study of genetic diseases: determination of inborn errors of metabolism through DNA analysis by means of specific probes able to bind to a specific gene or the study of enzymes and the characteristics of some metabolites for the diagnosis of some important metabolic diseases.

Benefits

The earlier sampling and the outcome of the chorionic villus examination compared to amniocentesis – about 5 weeks earlier – allows the woman to know early in pregnancy if there are fetal abnormalities. In this case the woman can decide for a possible termination of the pregnancy. Procedures aimed at terminating pregnancy at this gestational period are easier and safer, leading to a lower risk of complications from both a physical and psychological point of view. the short time needed to obtain results (7-10 days against the 2-3 weeks necessary for amniocentesis), it is especially recommended when:

• a mother aged over 38-40 years;
• there have already been previous pregnancies with fetuses affected by chromosomal pathologies;
• one or both parents are carriers of a balanced chromosomal abnormality;
• a faster response is preferred for psychological reasons.

The possible risks:

• failure of the intervention: a sufficient amount of chorionic material is not collected and the intervention must be repeated;
• abortion caused by villous sampling (from 0.5 to 1% depending on the case);
• possibility of onset of intrauterine infection with maternal clinical symptoms and sometimes with abortive outcome of pregnancy;
• maternal-fetal haemorrhage with consequent isoimmonization by Rh factor (with Rh negative mother, anti D immunoglubulins are administered);
• rupture of the membranes which, if it does not cause the onset of an abortion, can cause the formation of amniotic bridle with consequent formation of fetal compression anomalies;
• formation of subchorionic hematomas;
• discrepancy (in 1% of cases) between the karyotype obtained from the villi (which has a mosaicism) and the real karyotype of the fetus which is normal (this possibility occurs more frequently in the case of rapid preparations than in prolonged cultures);

There have been some rare reports of limb and face abnormalities attributable to chorionic villus sampling. It has never been statistically demonstrated an increase in these anomalies in newborns after villous sampling and in any case we perform these samplings at the end of embryogenesis, that is, after 11 weeks of pregnancy.

For many years it was feared and some still believe that the higher percentage of abortions and the relative technical difficulty of carrying out the chorionic villus sampling made this examination usable only as a second level examination for pregnancies at particular risk in a few specialized centers. According to recent controlled studies it seems that the risk of abortion from chorionic villus sampling is equal or even lower than the risk of abortion after amniocentesis if performed by expert hands.

The technique

Chorionic villus sampling can be performed outpatient transcervically or transabdominally between the 11th and 13th week of pregnancy.

Laboratory techniques and outcomes

The villi taken are examined with the direct method and then placed in culture for indirect examination. This allows to exclude any false positives, that is, falsely pathological tests for a laboratory artifact. In rare cases (1%) it is necessary to repeat the examination with chorionic villus sampling if the material is insufficient or with amniocentesis and further other tests in the event of an uncertain outcome.

Anti-D prophylaxis and fetal haemolytic disease

It consists of an intramuscular administration of anti-D antibodies. It is done to all pregnant women with negative Rh factor and indirect Coombs test (TCI) negative after amniocentesis, chorionic villus sampling that is after all invasive operations, as well as after childbirth or after an abortion.

After prophylaxis, the Coombs test (TCI) always becomes positive and generally remains positive for about 2 months. The Coombs test is used to measure anti-D antibodies which are administered with prophylaxis or which, in the rare cases of Rh isoimmunization, are produced by the pregnant woman herself. These antibodies administered to the pregnant woman destroy the red blood cells of the fetus which can be passed into the maternal blood and prevent maternal immunization against the Rh factor. If the pregnant woman herself produces antibodies to D, they pass the placental barrier and can destroy the red blood cells in the fetus causing, as the pregnancy progresses, a fetal anemia which in some cases will have to be treated with intrauterine transfusions.

For Rh negative pregnant women, signed informed consent will be requested. In some very rare cases a complication of prophylaxis has been found, consisting of an allergic reaction to the administration of immunoglobulins. Cases of contamination of immunoglobulins with the AIDS virus and hepatitis C have been very rare.