PRE-NATAL CHROMOSOME ANALYSIS

Pre-natal diagnosis is normally recommended for those persons at increased risk.

Parents are often anxious about the condition of their unborn child. Some parents already have a child with a birth defect and may wish to test if the same condition is present in a subsequent pregnancy. While the majority of newborns are physically normal, 3 to 5 babies out of 100 births may have birth defects, and some of these defects are due to chromosomal abnormalities. Approximately 0.5% of all newborns have a chromosomal abnormality. Knowing the chromosomal status of their baby early in the pregnancy allows the parents to have a chance to plan, prepare and make definitive decisions.

Who needs prenatal screening for chromosomal abnormalities?

All expectant mothers can be tested during their pregnancy for chromosomal abnormalities. But the risks of invasive prenatal testing may outweigh the benefits, so prenatal diagnosis is normally recommended for those persons at increased risk.

Who are at risk?

Pregnant women with:

  • Advanced maternal age

  • Abnormal maternal serum markers

  • Abnormal ultrasound markers (e.g. abnormal nuchal thickness in the foetus)

  • Recurrent spontaneous abortions

  • Known parental chromosomal abnormality

  • Known or suspected chromosomal abnormality in a previous child

At present, the conventional karyotype analysis still remains the gold standard for the evaluation of chromosome number and structural anomalies in pre-natal diagnosis.

How are chromosomal abnormalities in babies before birth diagnosed?

A diagnostic test such as amniocentesis or chorionic villus sampling can be recommended to confirm or exclude the presence of chromosomal anomalies. Before the test is done, it is important for the couple to consider how they might be affected if the test were to actually be positive for a chromosomal abnormality.

I. Amniocentesis

A procedure in which 20ml of amniotic fluid is usually extracted from the expecting mother under ultrasound guidance. Amniocentesis is performed between 15 to 18 weeks into the pregnancy. The cells are grown and processed in the laboratory for chromosomal analysis.

II. Chorionic Villus Sampling (CVS)

This procedure involves taking a small quantity of placental tissue between 9 to 12 weeks of pregnancy. The tissue is carefully processed in the laboratory and cultured for chromosomal analysis.

III. Cordocentesis

Fetal blood is obtained around 19 weeks of pregnancy under ultrasound guidance and cultured for chromosomal analysis.

OTHER TECHNIQUES AVAILABLE

Quantitative Fluorescent Polymerase Chain Reaction (QF-PCR)

This is the latest technique available for rapid diagnosis of the major chromosomal abnormalities, and can provide results (on the average) within 24 – 48 hours upon receipt of samples. The test still involves the invasive procedure of amniocentesis but requires much less amniotic fluid than conventional karyotyping. Usually the amount required is about 3-5ml of amniotic fluid. However, it is usually advisable to still go for a full karyotype investigation to exclude other rarer forms of chromosomal abnormalities not looked for by amnio-PCR.

Fluorescent in situ-hybridisation (FISH)

This approach also enables the detection of the major chromosomal abnormalities involving chromosomes 13, 18, 21, X and Y within 24 – 48 hours. Although generally more expensive than amnio-PCR, it is a useful tool in investigating chromosomal rearrangements and identifying unknown chromosomal pieces. It is particularly helpful for confirming abnormal chromosome numbers where a mix of both normal and abnormal chromosomes are present (mosaic cell lines).