Non-invasive prenatal diagnostic tests in pregnancy are being developed which could significantly reduce the number of pregnant women undergoing invasive tests like amniocentesis and chorionic villus sampling (CVS). Such tests carry around a 1 per cent risk of miscarriage, so each thousand invasive tests avoided would potentially save ten healthy babies. With 25,000 invasive tests for Down’s syndrome and 1,500 invasive tests for genetic disorders per year, the potential to avoid risk to the fetus and anxiety to the family is significant.
Lyn Chitty Professor of genetics and fetal medicine, UCL Institute of Child Health, is leading a five year research project to evaluate and, if appropriate, develop an implementation plan for some aspects of Non Invasive Prenatal Diagnosis. The multi-centre programme is called RAPID (Reliable accurate prenatal non-invasive diagnosis) and is funded by the National Institute for Health Research (NIHR).
The tests are based on cffDNA, cell free fetal DNA from the developing baby, which can be found in the mother’s bloodstream. Tests take a blood sample from the mother, centrifuge it, and then find ways to analyse the baby’s DNA.
This test is already offered for clinical use for sex determination in families at high risk of sex-linked inherited diseases, and for identifying whether the babies of RhD- mothers are RHD+ or RHD-. It has also been used in a very small number of cases to identify specific single gene disorders inherited from the father.
This test looks for a specific gene on the Y chromosome (the mother will not have any Y chromosome). It can be used to help management of the pregnancy, for example in children with a risk of inheriting congenital adrenal hyperplasia (CAH), it can reduce the need for steroid treatment in the womb. For children with inherited disorders like haemophilia, it is helpful to know the sex of the child to plan the delivery, since only boys will carry the disease The test is 98 per cent reliable from seven weeks and can be checked by ultrasound from around 12 weeks. Studies have shown that use of NIPD for the above conditions almost halved the number of mothers needing to undergo invasive testing, for these conditions.
RHD (Rhesus D) status
RhD- mothers are given a blood product Anti-D to ensure that their second and later children do not suffer haemolytic disease of the newborn (this arises when antibodies in the mother’s blood attack the developing baby’s blood and make it anaemic). Anti-D is a human blood product that is expensive to produce. If an RhD- woman is carrying a D- baby she does not need to have anti-D. Research is ongoing to determine whether it is possible to accurately test D- women routinely to see if they are carrying a D- or D+ baby. Around 105,000 pregnancies a year are in RhD- women, 15 per cent of the total in the UK.
Proof of concept research in small numbers of cases has shown that Down’s syndrome and other trisomies can be identified from around 11 weeks. However the reliability and cost effectiveness need to be rigorously assessed in much larger studies, and suitable laboratory and clinical standards drawn up before this could be used clinically. Dr Lyn Chitty said: “This has to be evaluated properly but, if these early results can be confirmed in larger trials, we are still at several years off a reliable non-invasive test for Down’s syndrome.”
ASSISTANCE FOR FAMILIES
Non-invasive testing for sex determination in sex-linked disorders, or RHD status in high risk women are available in many units. If you have any queries about these tests you should discuss any concerns directly with your consultant obstetrician or geneticist. Non-invasive prenatal testing for Down’s symdrome is not available anywhere so far as we are aware and the research team cannot offer this test. Discuss any concerns you may have directly with your GP or consultant, but they will not be able to commission a test from us. More information can be obtained from www.safenoe.org
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