CLINICAL DIAGNOSIS

CHROMOSOME SPECIFIC

Clinical Diagnosis

AMNIOCENTESIS

Overview

Amniocentesis is the most common test used for prenatal diagnosis of a chromosome problem in the baby.  It involves the removing a small amount of amniotic fluid which surrounds the baby in the amniotic sac. Amniocentesis is done after 15 weeks of pregnancy.  During the procedure ultrasound is used to locate the baby and the placenta.  A thin needle is inserted through the mother’s abdomen into the amniotic sac to remove some of the fluid surrounding the baby.  Within the fluid are cells which the baby has shed from its skin and bladder. The sample is taken to the lab and the baby’s cells are separated from the fluid.  The cells are grown in the lab and then examined under a microscope.  Results on the karyotype of the baby are received in two to three weeks.  There is a 0.5-1% risk of miscarriage above the background risk associated with the procedure.  That is, 1/200-1/100 women will lose the pregnancy due to complications of the procedure.

Prenatal Diagnosis

   Chorionic villus sampling

>Amniocentesis

   Ultrasound

   Confined mosaicism

   Uniparental disomy

Diagnosis in blood

Preimplantation diagnosis

The amniocentesis sample is sent to the laboratory where the cells are cultured (allowed to grow and divide) and then the chromosomes are analyzed.  In 12% of amniotic fluid samples that are analyzed more than one cell type is revealed (Chernos, 1994).  Usually the mosaicism is not reflective of true fetal mosaicism.  The frequency of confirmed chromosome mosaicism detected in amniotic fluid samples is about 0.1% (Bui et al, 1984).

As with CVS, 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}.

Some extra scientific details of mosaicism detection: link.

When chromosomal mosaicism is not detected in amniotic fluid, the abnormal cells are thought to be confined to the placenta.  Confined placental mosaicism is not of genetic significance to the developing baby. (Goldberg & Wohlferd, 1997)  Link to confined placental mosaicism.

Amniocentesis reveals a mosaic finding in 0.1% of all pregnancies.  Generally, the presence of the trisomy in amniotic fluid suggests that at least one fetal tissue may be affected by the trisomy.  However, it is possible to have high levels of the trisomy in amniotic fluid with no trisomy cells detected in blood or skin of the baby at birth.  There have also been cases in which the amniocentesis result was normal yet fetal mosaicism was present at birth (Phillips et al, 1996).

When mosaicism is detected on amniocentesis fetal blood sampling can be offered, where the expertise is available, to examine cells in fetal blood.  If the same abnormal cell line is present in fetal blood, fetal mosaicism would be confirmed.  When chromosomal mosaicism is confirmed in the fetus, a couple is presented with a very difficult situation, since fetal abnormality is not a certainty and clinical presentation may be variable.  On the other hand, if the fetal blood sampling result is normal the chance of mosaicism in the fetus is greatly reduced.  However, abnormal cells may still be present in tissues other than fetal blood.  These cases are difficult since fetal mosaicism can never be entirely excluded (Hahnemann & Vejerslev, 1997)

In either case, detailed serial ultrasound examinations may provide some reassurance if there is normal growth and no fetal anomalies detected.  

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