Screening tests can be defined as “a test carried out on a large number of apparently healthy people to separate those who probably have a specified disease from those who probably do not.” 
Antenatal screening is used to separate pregnancies into high and low risk groups for various conditions, after which women who are at high risk can be offered further diagnostic testing such as amniocentesis or chorionic villus sampling (CVS) when Down syndrome is positively screened for. The aims of such testing are as follows:
Screening in pregnancy aims to detect conditions in both mother and foetus. As early detection allows more time for decision making and early intervention, tests are carried out as soon as they are reliable enough.
Foetal screening is conducted in order to identify two possible complications of pregnancy: Structural Anomaly and Down's Syndrome.
Between 18 and 20+6 weeks gestation, every woman should be offered an anomaly scan. This involves measurements of the skull, brain and long bones of the foetus, imaging of the face, spine and organs and measurement of the amount of amniotic fluid. The position of the placenta is also recorded. At this point it is possible to see whether the foetus is male or female.
At this point, if any anomalies are seen the mother may be referred for counselling and diagnostic testing. At this stage of the pregnancy, the diagnostic test of choice would be amniocentesis.
All women should be offered screening for Down's syndrome, but it must be made clear that this is not compulsory. It is now recommended that all screening for this condition be completed by the end of the first trimester (13 weeks 6 days), although it is possible to test later in pregnancy for those women who book late. There are two methods by which Down's syndrome can be screened for. The recommended test is the combined test, which involves three components:
These markers are measured between 11 weeks 0 days and 13 weeks 6 days. Nuchal translucency measurement is also now recommended for use to screen for neural tube defects, replacing the alpha-fetoprotein serum testing.
If women book later than this, or it is not possible to assess nuchal translucency due to foetal position or maternal body habitus, the triple or quadruple test is used between 15 weeks 0 days and 20 weeks 0 days, which measure maternal serum markers:
The detection rate of the combined test is approximately 82%, the triple test 65% and the quadruple test about 74% for a 5% false positive rate. Results from Down's syndrome screening are given as a risk ratio relative to the woman's risk based solely on her age (for instance, at age 40 the age-related risk of Down's syndrome is 1:100). The threshold for diagnostic testing is 1:150, above which the woman is referred for counselling for diagnostic testing.
Counselling well for antenatal screening is of vital importance. Parents need clear, easy to understand and accurate information when making the decision of whether to consent to screening. As the decision to undergo foetal screening can have such profound effects on the family involved, it is vital to ensure that the decision made is as well-informed as possible and that the parents are happy with their decision.
When explaining screening to parents, the important information points to communicate are as follows:
It is also important to remember when counselling parents for screening that assumptions must be avoided: not all families will want to undergo screening but may feel pressured if it seems to be assumed that they will want it, and vice versa. Parents should be reassured that many people undergo screening tests and use the results to prepare for the arrival of an affected child. A positive screening test (and positive diagnostic testing) does not mean that they should or must have a termination, it merely provides them with more information.
While diagnostic testing is not technically part of antenatal screening, it is important to be able to discuss with parents what options are available to them following a positive screening result. The first option available is, of course, to do nothing further. Some parents may wish to stop at this point and prepare themselves with the information they already have.
However, other parents may wish to continue on to diagnostic testing. The tests most commonly used for this purpose are Chorionic Villus Sampling (CVS) and Amniocentesis, both of which involve sampling the foetus' DNA. The Royal College of Obstetricians and Gynaecologists estimates that 5% of pregnant women are offered these tests each year.  Each test makes it possible to determine a karyotype for the foetus and therefore to detect chromosomal abnormalities.
A sample of the amniotic fluid is aspirated by passing a needle through the abdomen into the uterus, using ultrasound guidance. It is the most commonly performed antenatal diagnostic procedure performed, due to the lower miscarriage risk and later gestation at which it is generally undertaken (from 15 weeks onwards).
The additional miscarriage risk following amniocentesis is 1%. Early amniocentesis (before 15 weeks gestation) is not recommended as it is associated with a higher incidence of talipes and foetal loss.
This procedure involves sampling the placental villi and is performed between 11+0 weeks and 13+6 weeks gestation. It can be achieved using either a transabdominal or transcervical approach. The earlier gestation at which CVS can be performed means that it is useful when earlier diagnosis is desirable.
The additional miscarriage risk for CVS (i.e. the risk of miscarriage added by CVS on top of the baseline risk of miscarriage associated with any pregnancy) is slightly higher than that of amniocentesis; an important consideration for parents who may be willing to wait until a later gestation.
When consenting for either of these tests, it is important to explain that there is a risk of miscarriage associated with the test. There is also a risk of introducing infection (as with any procedure which introduces a foreign item into the body, past the natural protections of the body), of transmitting infection from mother to foetus, of pain or adverse reaction to the procedure on the part of the mother (vaso-vagal attacks can be triggered) and also that the procedure may fail.
Other information to share is the length of time parents may have to wait for results: typically this may be a few days for a provisional result and a couple of weeks for final karyotyping results.
For a more detailed explanation of antenatal diagnostic tests, please see the following article: here.
NICE: Antenatal care: routine care for the healthy pregnant woman (2008) Available at: http://guidance.nice.org.uk/CG62
Royal College of Obstetricians and Gynaecologists: Amniocentesis and Chorionic Villus Sampling (2010) Available at: http://www.rcog.org.uk/files/rcog-corp/GT8Amniocentesis0610.pdf
UK National Screening Committee: Fetal anomaly screening programme - screening for Down's syndrome: UK NSC policy recommendations 2007-2010: Model of best practice (2007) Available at: http://fetalanomaly.screening.nhs.uk/cms.php?folder=2454
 Oxford Concise Medical Dictionary, 6th Ed., University Press 2003.
 Royal College of Obstetricians and Gynaecologists, Amniocentesis and Chorionic Villus Sampling (2010), p. 2.
 Image courtesy of the National Human Genome Research Institute. Available at: http://www.genome.gov/Glossary/index.cfm?id=114
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