![]() ![]() This is the most common cause of intellectual disability. Other signs seen on the scan, known as 'soft' markers can also be used to modify the results of Down's syndrome screening but the scan cannot be used to exclude this disorder or Edwards' syndrome. In most countries a specialist ultrasound ('anomaly') scan is carried out routinely at 18-20 weeks gestation to identify structural abnormalities in the fetus. For FXS, SMA and CF results are available within days. Occasionally, a conclusive result is not obtained and the procedure needs to be repeated. With CVS, a provisional result may be given within 1-2 days however, it may take 2-3 weeks to confirm. In some cases other mild complications occur such as bleeding or leakage of amniotic fluid.įor chromosomal anomalies, results are usually available within 2-3 weeks. Both procedures carry an element of risk and about one woman in every 100-200 may miscarry, over and above those who would have miscarried anyway. The advantage of CVS is that it can be done earlier in pregnancy, from about 10 weeks amniocentesis is usually performed around 16 weeks, although practice varies. Some women may experience slight discomfort. A sample is taken of the placenta which comprises fetal cells only.īoth procedures take about 10 minutes and are performed on an out-patient basis. CVS can be done either by the insertion of a fine needle through the abdomen or through the vagina. A sample of the amniotic fluid is taken and from this fetal cells are extracted. Amniocentesis can also be used in the diagnosis of spina bifida and AWDs by measuring the concentration of alpha-fetoprotein (AFP) which leaks from an open fetal lesion into the fluid surrounding the baby (amniotic fluid).Īmniocentesis involves inserting a fine needle through the woman's abdomen into the womb. *These tests are not generally available on the NHS but can be obtained privatelyĪmniocentesis and CVS are used to obtain fetal cells which can be analysed for chromosome number and genetic mutations. The gestational age at which the various screening and diagnostic tests are carried out and the type of procedure involved are: The detection rate is the percentage of affected pregnancies identified as high risk, and the false-positive rate is the percentage of unaffected pregnancies identified as high risk. ![]() The quality of a screening test is determined by the 'detection rate' and the 'false-positive rate'. It does not mean that an affected pregnancy has been completely excluded. Similarly, a screen negative result means that there is not a high risk. Most of those with screen positive results go on to have normal babies. It does not mean that the baby is definitely affected. A screen positive result means that the risk is high enough to consider having prenatal diagnosis. The results of a screening test are either 'screen positive' or 'screen negative'. † in the 1970's: current rates unknown but thought to be lower because of dietary changes. The table shows that most of the additional disorders are either rare, incompatible with survival, or surgically correctable: These are Edwards' syndrome, and if testing is after 15 weeks gestation, Neural Tube Defects (NTDs) and Abdominal Wall Defects (AWDs). In addition the Down's syndrome screening tests can also detect other disorders. Screening tests available today are designed principally to screen for Down's syndrome, fragile X syndrome (FXS), Spinal Muscular Atrophy (SMA) and Cystic Fibrosis (CF). Following this, if the pregnancy is found to be affected a further decision will need to be made: whether to have a termination of pregnancy. For those screened and identified as high risk, there is the decision whether to undergo prenatal diagnosis. ![]() Initially there are three: whether to be screened at all, for which specific disorders, and which test. Those considering screening need to make a number of decisions. Screening does not replace diagnosis it aims to provide information which can help decision-making. The main procedures are amniocentesis and chorionic villus sampling (CVS). Selection is needed, since for most disorders diagnosis is only possible by an invasive procedure and this carries a slight risk of miscarriage. Screening is used to select a high risk group so that they can be offered prenatal diagnosis. Prenatal diagnosis establishes whether or not the disorder is definitely present. Howard Cuckle (UK) Screening differs from diagnosisĪntenatal screening is the process of identifying those at high risk of a disorder. ![]()
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