CLINICAL DIAGNOSIS

CHROMOSOME SPECIFIC

Clinical Diagnosis

CHORIONIC VILLUS SAMPLING (CVS)

Overview

Chorionic villus sampling (CVS) is an alternative to amniocentesis for prenatal diagnosis. CVS is a prenatal testing procedure offered in certain pregnancies for determination of the karyotype of the fetus.  The chorionic villi are part of the placenta.  CVS involves removing a small sample of the placenta with a thin needle which is inserted through a woman's abdomen or with a catheter which is inserted through the vagina and cervix. The location of the placenta sometimes dictates which method is used.  The procedure is carried out under the guidance of ultrasound.  In most pregnancies the chromosomal content detected in the placenta is an accurate representation of the chromosomes in the fetus.  CVS is performed at 10-12 weeks gestation.

Prenatal Diagnosis

>Chorionic villus sampling

   Amniocentesis

   Ultrasound

   Confined mosaicism

   Uniparental disomy

Diagnosis in blood

Preimplantation diagnosis

The placenta is the connection between the mother and her baby.  It allows substances to pass from the mother to the baby and from the baby to the mother. It also produces hormones which help support the pregnancy.  The fetus and the placenta both develop from the same fertilized egg or zygote.  Thus, the chromosome content in the fetus and the placenta are usually the same.  In the earliest stages of development, the single-celled zygote undergoes multiple cell divisions.  As the cells continue to divide, some start to differentiate, that is they become programmed to develop into a certain cell type.  The first cells to differentiate are the trophoblast cells, which are destined to become part of the placenta.  The trophoblast cells are involved in implantation of the embryo in the wall of the uterus.  In the fully developed placenta, the trophoblast cells are the outer layer of the chorionic villi.  Other non-fetal cells become the villus stroma or mesenchymal core.  Here's an illustration of the placenta and the baby.

Tell me more about the development of the placenta. (more scientific details)

The CVS sample is sent to the laboratory where it can be examined either immediately, or after the cells are cultured and allowed to grow and divide.

1. Direct analysis is done immediately.  This type of analysis examines the trophoblast cells of the placenta.  Trophoblast cells are very rapidly dividing cells, which enables them to provide tissue to attach to the uterine wall.  This rapid division may bring on a greater vulnerability to mitotic error (Gardner & Sutherland, 1996).
2. Cultured analysis is done on cultured cells.  This type of analysis examines the fibroblast like cells of the villus stroma or mesenchymal core found in the villus structure of the placenta.  It is thought that this method more accurately reflect the chromosomes of the fetus.  This is because the cells which become the villus stroma is more closely related to the cells which become the embryo, based on early embryonic development.

If it is not specified whether CVS was preformed on ‘direct’ or ‘cultured’ cells it usually means that cultured cells were used.

True chromosomal mosaicism is when two or more cells lines are detected in two or more culture flasks from the same individual.  Pseudomosaicism is a term used to describe two cell lines that are found in only one culture flask.  Pseudomosaicism is not concerning as it is generally a result of cultural artifact and not representative of a true finding in the baby. It is therefore not normally reported to the patient {Commentary on pseudomosaicism reporting}.

Approximately 1-2% of CVS results cannot be interpreted because of the presence of two cell lines in the placenta with different chromosomal complements (chromosomal mosaicism). When this happens the patient is offered the option of an additional prenatal diagnostic procedure (amniocentesis or fetal blood sampling) in an attempt to clarify the results.  

The finding of mosaicism on CVS always requires careful evaluation of the pregnancy as a whole, to try to determine if the abnormal cell line is also present in the fetus or if there is a risk of fetal uniparental disomy (Hahnemann & Vejerslev, 1997).  

The result of chromosomal mosaicism on CVS can mean one of four things:

1. the trisomy cells are only in the placenta and there will be no harmful effect on the development of the baby or the ability of the placenta to function properly.  These pregnancies will progress normally.
2. the trisomy cells are only in placenta and there are no genetic concerns with the development of the baby, however, the presence of the abnormal cells in the placenta impairs its ability to function properly.  Some of these babies may be small (IUGR), may be delivered prematurely, and in rare situations the impaired placenta may cause loss of the pregnancy.  Link to confined placental mosaicism.
3. the trisomy cells are only in placental tissue and normal diploid cells are found in the baby.  However, a closer look at the chromosomes in the baby show uniparental disomy.  Uniparental disomy for certain chromosomes is associated with health concerns.  Link to uniparental disomy.
4. the trisomy cells are both in the placenta and in the baby.  Mosaic cells in the baby have a variable effect on growth and development.

It is very important to know that an abnormal CVS result does not mean that there are trisomic cells definitely in the baby. In fact, in most cases the abnormal cells are not suspected to be in the baby or to effect the health of the baby.  This result also does not mean that there will definitely be a problem with the ability of the placenta to function. 

CVS mosaicism is a very stressful result for expecting parents.  When mosaicism is detected on CVS, couples are presented with several options to try to determine if the baby is affected; invasive prenatal options of confirming the mosaicism on amniocentesis or on fetal blood sampling.  The other option is to sample fetal skin cells at birth.

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