| Until now, prenatal diagnosis of Downs | | | | (13 weeks or below). |
| syndrome (Trisomy 21) for the unborn baby has | | | | |
| required an invasive test. This has taken the | | | | The breakthrough was that they managed to |
| form of an amniocentesis where fluid is taken | | | | successfully identify and separate the small |
| from around the fetus in the womb after 14 | | | | amount of fetal DNA that cross into the |
| weeks of gestation. The alternative, | | | | maternal circulation. This was analyzed via |
| available about 3 weeks earlier is CVS where | | | | what are known as SNP (single nucleotide |
| a tiny piece of the afterbirth (placenta) is | | | | polymorphisms) tests to identify where there |
| extracted. | | | | was an extra chromosome on set No. 21 |
| | | | (Trisomy 21). |
| Screening Tests | | | | |
| | | | Their tests in this study correctly |
| The screening tests which are non-invasive | | | | identified 58 of the 60 chromosomal make up. |
| have varying degrees of detection rates from | | | | Among the 58 correctly analyzed, two had |
| a lowly 30% for maternal age alone to a | | | | Trisomy 21 (Downs). The two that were not |
| clearly impressive 85% for a combination of | | | | correctly analyzed included one who had Downs |
| maternal age, fetal nuchal translucency | | | | but labeled as normal (false negative) and |
| (scan) and maternal serum biochemistry at | | | | the other one was incorrectly identified to |
| 11-14 weeks. This is called the Integrated | | | | have Downs syndrome (false positive). |
| Test. | | | | |
| | | | What does this imply? |
| Research has also shown that fetuses affected | | | | |
| by Downs syndrome have a missing nasal bone | | | | This is potentially quite a major advance. |
| on an ultrasound scan performed at 11-14 | | | | Its strength lies in the fact that it does |
| weeks. This test alone has been shown to | | | | away with an invasive procedure therefore |
| identify around 70% of Downs syndrome babies. | | | | eliminating in a stroke the spectre of |
| Crucially however, it did not appear to give | | | | miscarriage that hangs over the currently |
| any advantage over the longer established | | | | used procedures. This risk is indeed small |
| nuchal translucency scan performed at the | | | | but quite real and when it occurs, is |
| same gestation and therefore the uptake has | | | | devastating. |
| been poor. | | | | |
| | | | However, it is important to stress that this |
| One of the most frustrating weaknesses of all | | | | is only an early stage of this potential |
| screening tests is the fact that they also | | | | breakthrough. Here are the facts: |
| falsely detect unaffected babies, | | | | |
| occasionally leading to unnecessary invasive | | | | The study had quite a small sample |
| diagnostic tests and always causing | | | | (60) and it is therefore crucial that it is |
| significant stress to prospective parents. | | | | replicated in larger studies. |
| The false positive rate of screening tests is | | | | |
| around 5%. | | | | There is still a lot of room for |
| | | | refinement. In this study, whilst the |
| Moreover, even the most sensitive screening | | | | earliest sample was taken at 8 weeks |
| test will still miss some babies that are | | | | gestation, the first case of Downs was |
| affected and for all parents this means a | | | | identified at just over 17 weeks. For this to |
| little seed of doubt until the baby arrives. | | | | be truly useful, it will have to have the |
| | | | ability to establish the diagnosis early, |
| Diagnostic Tests | | | | ideally in the first trimester. Also the |
| | | | interval between blood sample collection and |
| The definitive diagnostic tests mentioned | | | | getting the results need to be reasonably |
| (amniocentesis and CVS) suffer from one | | | | short, lasting no more than 2 or 3 weeks. |
| Achilles heel: They are invasive and | | | | |
| therefore carry the potential risk of | | | | The current diagnostic tests have |
| provoking miscarriage. This can happen | | | | quite a high 99% accuracy rate. This test |
| regardless of whether the baby is affected by | | | | will need to match that and ideally surpass |
| Downs syndrome or not. Because of this, there | | | | it. |
| has been a lot of research activity to find a | | | | |
| risk-free reliable diagnostic prenatal test | | | | Science research thrives on this kind of |
| for Downs Syndrome. This is why the recent | | | | challenge and there is a lot of excitement in |
| announcement of a development of such a test | | | | the medical research world about this |
| reported in the medical journal The Lancet | | | | potential breakthrough. |
| (Feb. 3, 2007) has caused such a stir. | | | | |
| | | | In fact, this research work by Ravgen Inc. in |
| Maternal Blood Test | | | | the United States is not the only current |
| | | | development in this area. Researchers in Hong |
| Dr Ravinder Dhallan of Ravgen Inc. has | | | | Kong are also reportedly on the verge of a |
| reported results of a study to diagnose Downs | | | | similar breakthrough but employing a |
| syndrome for the unborn baby using Maternal | | | | different method which utilises placental DNA |
| blood and therefore practically eliminating | | | | in maternal circulation. |
| the potential risk of miscarriage associated | | | | |
| with the available invasive tests. What Dr | | | | These methods might usher in a new era of |
| Dhallan and colleagues did in this study was | | | | safe and effective prenatal diagnostic tests. |
| to collect blood from 60 mothers whose median | | | | If the hurdles are overcome, the rough |
| age was 34. Eight of the mothers had their | | | | estimate is that the tests may be available |
| blood samples taken in the first trimester | | | | for use in about three years. |