You Ask , They Answer
Q
: When would prenatal screening usually take place for conventional pregnancies ?
A
: Normally , we would screen for chromosome problems between 11 and 14 weeks . This is first trimester screening , when we would measure nuchal translucency to determine the thickness of the unborn baby ’ s neck and look for the presence of a nasal bone as both of these are markers for Down ’ s syndrome and other genetic problems .
In recent years , what has become quite popular is noninvasive pregnancy testing which takes a sample of the mother ’ s blood and assesses the placenta ’ s DNA . This can tell us the presence of Down ’ s syndrome in 99 percent of cases .
In high-risk cases , we will offer prenatal diagnostic tests such as amniocentesis and chorionic villus sampling where a small sample of cells is taken from the placenta . This has almost 100 percent accuracy in identifying the presence of abnormalities .
Q
: Is prenatal testing done as a matter of course ?
A
: Non-invasive pregnancy testing has become popular only over the last three to five years , while first trimester screening has been available for about a decade . Procedures like amniocentesis have been around since I ’ ve been practicing obstetrics .
Based on best practices , we should always discuss Down ’ s syndrome and chromosome screening with patients who are almost all offered the opportunity to screen for chromosomal abnormalities at 11-14 weeks , and there will always be a 20 weeks foetal abnormality scan . That is the standard of care , and almost all patients , at least in my practice , will take up some form of screening . Rarely do we have patients who will overlook genetic screening at 11-14 weeks .
Q
: When are prenatal diagnostics performed ?
A
: Screening tests are considered general tests that are inexpensive and non-invasive , so there is no risk to the mother or foetus . However , these tests may show false positives . In other words , the foetus may not be afflicted with the disease in question , but screening still shows positive . With this group , we will perform diagnostic tests to confirm the diagnosis . A good screening test will have a reasonable rate of false positives , but hopefully we ’ ll catch most of the true positives in the screening test . The mother could experience a minor scare during the testing process , but the important thing is that we won ’ t miss some of the truly positive cases .
Q
: What is preimplantation genetic testing in IVF pregnancies ?
A
: The crux of IVF is to always identify an embryo worthy of transfer — the best embryo . We would previously rely on morphology , or the appearance and growth of the embryos up to day three . But in recent years it ’ s become popular to allow the embryos to culture up to day five — what we call blastocysts — and at that point , genetic testing may be performed . This involves taking a sample of the cells to screen for chromosomal abnormalities . After screening , we only transfer the chromosomally normal embryos , which hopefully improves the implantation rate and chance of a healthy live birth .
Whether this can be performed , however , is dependent on legislation in different countries . In Singapore , legislation to allow preimplantation genetic testing for use as a general routine procedure is still under discussion . The government is evaluating the pros and cons , such as its cost-effectiveness . Right now there is a large-scale study being performed to assess the effectiveness of preimplantation genetic testing in patients over 35 who have previously miscarried . The government is evaluating the results of this study before deciding if it should be allowed on a national level for prenatal care . Certainly preimplantation genetic testing is a useful strategy to have .
Dr Tan Heng Hao
Dr Tan Heng Hao is a consultant fertility specialist , obstetrician , and gynaecologist and also the medical director of Alpha IVF Centre in Singapore .
10 OCTOBER 2021 GlobalHealthAsiaPacific . com