Thomson Prenatal Diagnostic Laboratory

One of Singapore's first cytogenetics laboratories, Thomson Prenatal Diagnostic Laboratory specializes in the identification of chromosomal abnormalities such as Down Syndrome, Patau Syndrome and Edwards Syndrome. The laboratory facilitates for the rapid diagnosis of common chromosomal abnormalities by either quantitative fluorescent polymerase chain reaction (QFPCR) or fluorescent in-situ hybridization (FISH) testing. The Laboratory performs biochemical screening of maternal serum abortuses to detect Down Syndrome, Edwards Syndrome and neural tube defect. We also investigate for constitutional chromosome abnormalities in patients of all ages as well as in spontaneous. Other services include facilitation of molecular genetic tests such as prenatal thalassaemia genotyping, paternity testing, fragile X syndrome, Y microdeletion male infertility and several other genetic conditions. Prenatal Diagnosis 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

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.

1.	Amniocentesis A procedure in which 20ml of amniotic fluid is usually extracted from the expecting mother under ultrasound guidance. Amniocentesis performed between 15 to 18 weeks into the pregnancy. The cells are grown and processed in the laboratory for chromosomal analysis.
2.	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.
3.	Cordocentesis Fetal blood is obtained around 19 weeks of pregnancy under ultrasound guidance and cultured for chromosomal analysis.

What new laboratory techniques are available for analysis of genetic material?

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 of 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).