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Why Prenatal Diagnosis?

Every pregnancy is a precious pregnancy. In these days of small family size, there is a growing urgency for each baby to be born normal, not afflicted with any congenital defects or inheritable diseases. Prenatal investigations using PCR and gene probes are playing a pre-eminent role. Advance ultrasound capability and post-acquisition rendering and the use of soft markers also carve out new frontiers in prenatal screenng.

Fetal tissues have been sampled conventionally from the amniotic fluid (by amniocentesis)or placenta bed (by chorionic villus sampling CVS). The ability to identify, amplify and re-constitute genetic information using fetal DNA fragment from the mother's blood is a seismic paradigm shift.

What Is Amniocentesis?

Amniocentesis is the procedure of aspirating amniotic fluid from the amniotic sac. Around the 6th week of pregnancy, the amniotic sac develops with the fetus growing within it. The reasons for amniocentesis are mainly to diagnose genetic conditions (via karyotyping) and to evaluate intrau-uterine infection. The fluid is cultured for microbial organism that may have affected the normal formation of the fetal organs and the growth of the fetus. On the other hand, the fetal cells are cultured, so as to characterize the genetic identity. Not so long ago, it was normal practice for mother older than 35 years to have amniocentesis at mid-trimester to exclude Down’s syndrome. Besides Down’s syndrome, other genetic conditions can also be checked if a specific anomaly is suspected.

How Is Amniocentesis Performed?

Amniocentesis is usually performed after 18 weeks of gestation, when the amniotic fluid volume is large enough to allow the removal of 20ml of fluid, without untoward effect to the on-going pregnancy. Ultrasound is used to guide the insertion of a very fine needle that is used to aspirate the fluid. Amniocentesis usually takes about ten to twenty minutes with a post-procedure recovery period of thirty minutes.

What is Down's syndrome?

People with Down's Syndrome, although share certain typical traits, do vary in appearance, personality and ability. Some adult DS can handle simple jobs and live fairly independent lives, while majority need long term help and care. Besides varying degree of learning disabilities (Down's Syndrome average IQ is about 80), several health problems are associated with Down's Syndrome, and they include heart problems, reduced hearing, poor vision, and earlier Alzheimer’s disease for those who live to be 50 years.

How common is Down’s syndrome?

Any woman can have a baby with Down's Syndrome, but the risk does increase with age. The risk of a pregnant woman of having a Down's Syndrome is 1:1500 (0.07%) at 20 years old, 1:900 (0.1) at 30 years old and 1:100 (1%) at 40 years old.

What causes Down’s syndrome?

Chromosomes are tiny structures in our cells that carry genes that determine how we develop. We have 23 pairs of chromosomes in each cell, receiving one set of 23 chromosomes from each parent. When the parents mate, the fertilization of the egg by the sperm, requires the egg and the sperm cells to divide and re-arrange correctly. At times when the pairs of chromosome fail to divide correctly, it will cause the baby’s cells to have an extra copy of chromosome number 21, resulting in Down's Syndrome. Almost all Down's Syndrome occurs by the accident, and has nothing to do with either parent. Since we are not able to remove the extra chromosome 21 from the baby cells, there is no cure for Down's Syndrome. Reducing the Down's Syndrome burden.

Detecting if Down's Syndrome is present in a pregnancy and preventing its birth may be the only way to reduce the “burden” of Down's Syndrome. This burden is borne by not just the person with Down's Syndrome, but also the parents and the society at large. The decision whether to have early detection rests on the parents (especially the mother). There are pregnant women who do not wish to find out if the unborn baby has Down's Syndrome.

Confirming if Down's Syndrome is present in a pregnancy is by using invasive techniques like chorionic villus sampling and amniocentesis to obtain component cells from the unborn baby for genetic analysis. These diagnostic tests carry certain risk of miscarriage and therefore are not offered to all women.

In recent years, a group of screening tests have been developed to pick up women who are at high risk of carrying a Down's Syndrome baby. These screening tests do not have risk of miscarriage, and although they do not definitely tell us if the baby has Down's Syndorme, they indicate if the chance of the baby being one is high. When the screening test return a high risk result, the mother will be offered the confirmatory test. It is vital to understand that the screening tests cannot exclude if the baby definitely has or definitely has not Down's Syndrome. The sequential use of screening and confirmatory tests in high risk result is the current standard practice, and it is devised because there is no available risk-free diagnostic test at the moment.

Understanding the screening test results

The results of the screening tests is summarized in a risk score, ranging from a low risk score of 1:>10000 to a high risk score of 1: less than 50. If the risk score is lower than the recommended national cut-off, the baby is considered to be at low risk of having Down's Sydrome. A high risk result is when the risk score is above the national cut-off. The national cut-off is set at 1:200, because above this risk score, we are able to diagnose 95% of the Down's Syndrome babies.

A low risk result means exactly that the baby is unlikely to have DS, it does not mean the baby has no risk of having Down's Syndrome. Some risk is still present because the screening tests are not perfect. Some babies with Down's Syndrome are not detected because of natural variations affecting how they express their characteristics when the screening tests are performed. About 25% of babies with Down's Syndrome are not detected by screening tests.

About 1:30 (3%) of women who are screened returned with a high-risk result, and diagnostic test will be advised. Having a high-risk result does not imply that the baby definitely has DS. It means that the risk is significant to further evaluate. Appreciating that the risk is higher than 1:200, majority of those who go for the confirmatory test learn that their baby does not have Down's Syndrome.

Should l have the tests for Down’s syndrome?

Advancement in prenatal screening has certainly reduces the number of unnecessary invasive tests and therefore miscarriages. By recommending screening tests to majority of women, it also introduces stress to a wider community of pregnant women when each has to decide if they wish to take up the screening tests. You will need to balance between the perceived and real risk of the baby having Down's Syndrome, against the bother and the cost of attending the screening test. Unfortunately only you can decide. Hopefully, you will be in an easier position to decide when you have gathered enough information about the condition and the current practice of risk assessment.


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