Prenatal Diagnosis
on this page :- Introduction, Amnio-PCR
Introduction
Thank you for deciding to have your baby at the Ulster Hospital. At your first visit an ultrasound scan may be performed to determine if your pregnancy is progressing as expected. The scan may also show if you are carrying more than one baby.
Mothers are offered several screening tests which may be able to identify whether or not your baby has a major problem, such as spina bifida or Down’s syndrome. Before deciding whether or not you wish to have these tests, please read this carefully. If you have any questions or would like to discuss any of the issues in the leaflet, please talk to your doctor or your named midwife.
Screening
Overall, Down’s syndrome occurs about once in 900 births. This is a genetic problem where the baby has an extra chromosome. The chance of you having a baby with Down’s syndrome increases as you get older. An ultra sound scan, or a blood test from the mother, may help to decide the risk of carrying a baby with this problem. If you decide to have one or both of the above tests, you will be informed of your particular level of risk. If you consider this risk too high, you will then be offered an amniocentesis. This test will tell whether or not the baby has a chromosomal problem. Amniocentesis involves inserting a needle into the womb and withdrawing a small amount of fluid from around the baby. The risk of amniocentesis causing a miscarriage is generally accepted as one in one hundred.
The big scan
At your first visit a scan between weeks of your pregnancy will be arranged to check that your pregnancy is progressing normally, and to determine approximately when your baby is due to be born. The scan will show the baby’s development and will also sometimes detect a major abnormality such as spina bifida. Ultrasonic markers can also be noted at this scan. These are features that appear in some babies during their development and mostly are of little consequence. However their presence may suggest that the baby carries an increased risk of having a chromosome problem such as Down’s syndrome.
Points to remember
• These tests are voluntary. The choice of whether or not to have any test is a personal decision for you and your partner.
• When making your decisions think carefully about what you would do if a serious problem was found. It is possible that you might be faced with the choice of whether or not to continue with the pregnancy.
• We can detect approximately two out of every three major structural problems in babies, as well as some minor problems. However it is important to remember that a normal scan cannot give a 100% guarantee of a normal baby. For example ultrasound is not able to pick up the majority of Down’s syndrome babies.
Amnio-PCR
Amniocentesis results in 48 hours
Any pregnant woman who has been advised by her antenatal team (midwives, GP’s and consultants) to undertake an amniocentesis test will benefit from Amnio-PCR.
Background:
Down syndrome is associated with varying degrees of physical and mental retardation and is the commonest single cause of learning disability in children of school age. It occurs by the random addition of a chromosome 21 at conception, which happens more regularly in mothers of advancing years. However, regardless of maternal age, the incidence of Down syndrome is 1 in 600 births.
Down syndrome and other genetic abnormalities can be diagnosed following invasive procedures such as chorionic villus sampling and amniocentesis at a safe time during the pregnancy. CVS biopsies are taken around the eleventh week whilst an amniocentesis is normally performed between 15 and 20 weeks gestation by withdrawing amniotic fluid surrounding the fetus.
Invasive procedures are recommended to “at risk” patients and routinely sent to the laboratory for culture and analysis taking two to three weeks to complete. The waiting time for such an important result can sometimes be agonising for the mother-to-be.
Amnio-PCR provides a rapid diagnosis of the major chromosome abnormalities including Down syndrome, giving a definitive result on average within 48 hours.
Aims of Amnio-PCR:
• To provide a rapid (48 hours) and reliable antenatal diagnosis of the most common chromosome abnormalities that could result in birth defect.
• To reduce the time delay associated with cell culture.
• To alleviate anxiety by minimising the waiting time for a result.
• To enable a mother and her health care team to decide the most appropriate action at the earliest possible stage of pregnancy.
Advantages of Amnio-PCR:
• Only a small amount of amniotic fluid is required.
• Applicable to a wide range of pregnancies (12 to 34 weeks).
• 100% accurate in the detection of major autosomal trisomies.
• 99.8% of pregnancy outcomes correctly identified by Amnio-PCR.
• Concurrent testing for cystic fibrosis and fetal sex when required.
How Amnio-PCR works:
During the normal procedure for amniocentesis, fluid is taken from the amniotic sac, surrounding the fetus, under ultrasound guidance. As part of the normal growth process, fetal cells are released into the amniotic fluid. DNA (material inside a cell that carries genetic information) is extracted from the fetal cells found within the amniotic fluid specimen. Selected areas of this DNA are labeled with a fluorescent tag and amplified by PCR. These fragments are then run on a DNA sequencer and finally analysed by skilled scientists with the aid of a computer programme.
The specific chromosomes tagged identify Down syndrome and other serious birth defect anomalies including cystic fibrosis if required. This is all completed within 48 hours to help alleviate stress and anxiety often experienced in the wait for results.
Accuracy of Amnio-PCR:
Prior to developing Amnio—PCR assay, our in house data of
50,000 routine amniocyte cultures was reviewed to establish
(using a large series of pregnancies) what types and
frequency of chromosome abnormality might be expected.
In terms of birth outcome there were 49,110 normals plus
762 abnormals which could be identified by Amnio-PCR (99.74%) and 128 (0.26%) which could not.
Since that time over 5,000 samples of amniotic fluid have now been analysed in our laboratory by Amnio-PCR assay and the results are summarised below:—
• No false positive results
• 100% accuracy in the detection of Down, Patau and Edwards syndromes and of triploidy.
• 86% (762/890) of all clinically relevant abnormalities recognised by traditional culture methods were detected by Amnio-PCR assay.
• Results available within 48 hours.
Availability of Amnio-PCR:
This new test is not yet funded by the NHS so the cost of a rapid PCR assay (aprox £80 in the Ulster Hospital) must be borne by the patient. It is available under these terms and at low cost in many major obstetric units throughout the UK.(For appointments please call Maternity Outpatients 028-9048-4511)
A small sample of amniotic fluid is sent to our genetics laboratory using the kit provided by The Doctors Laboratory and a definitive result is then provided within 48 hours of receipt. The remainder of the specimen should be used for culture in the normal way to exclude very rare chromosome abnormalities not assayed for by Amnio-PCR.
Questions & Answers:
Q. Who can have an Amnio-PCR test?
A. Any pregnant woman who has been referred for an amniocentesis test can request Amnio—PCR.
Q. How do I decide whether to have an invasive amniocentesis test or no test at all?
A. This is a decision that you should make after discussions with your partner, your consultant and your midwife. Your antenatal team will normally provide you with all the information and counselling that you will need to make this choice. They should explain to you the implications of this test and the factors that you need to consider.
Q. How is the Amnio-PCR sample obtained?
A. A small part of the amniotic fluid sample collected is put aside to be sent to The Doctors Laboratory Genetics Division for analysis. The rest of your specimen will be taken to the local laboratory in the Belfast City Hopsital for routine cell culture. As an alternative, the entire sample may be despatched to us for PCR plus culture although the cost is greater.
Q. Is it true that I must pay for Amnio-PCR assay?
A. If you choose to have this test, yes. At the moment there is no general NHS funding to cover Amnio-PCR and an invoice will be send directly to you from The Doctors Laboratory.
Q. How do I obtain my results?
A. Your Amnio-PCR result is given to the antenatal team normally within 48 hours following receipt of the specimen at the laboratory. Any interpretation of the answer will be fully explained to you by your consultant.
Q. How can I be sure that Amnio-PCR is accurate?
A. Only a small proportion of patients (2 per 1000) have a rare abnormality which is not found by our AmnioPCR assay, simply because markers for these rarities are not included in the testing regime. They are, however identified upon culture later. Where Down syndrome and other major trisomy abnormalities are concerned the assay has proved 100% accurate. Consultant obstetricians worldwide use our services with confidence.
Q. What if my consultant wishes more precise information regarding this test?
A. The highly skilled scientists responsible for developing this test are available to discuss the principles involved with Amnio-PCR
Q. How am I able to find more information about AmnioPCR?
A. Your antenatal team should be able to respond to your request, however, you may wish to visit our website on www.amnio-pcr.com
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