Prenatal diagnosis involves screening or testing an embryo or fetus before birth. There are three main advantages of prenatal diagnosis:
There are many different methods of calculating the likelihood that a fetus had an abnormality depending on the gestation of the mother. These are not diagnostic tests, rather they calculate your risk of having a baby with certain abnormalities. Even if you are calculated low risk, it is still possible to have a baby with this disability. The screening available during pregnancy is listed in chronological order below.
Maternal blood test
A sample of blood is taken from the mother, usually at her booking visit. This is tested to determine the Sickle cell disease and Thalassaemia carrier status of the mother. If necessary, paternal blood is also tested. Hepatitis B, HIV and Syphillisare screened for as if detected, much can be done to prevent mother to baby transmission. Maternal blood group and rhesus status are also checked to prevent Rhesus disease of the newborn.
First trimester combined test (11-14 weeks)
This involves measuring levels of PAPP-A and free beta-hCG in the maternal blood, along with a nuchal translucency scan. These results are combined with maternal age and gestational age of the fetus to calculate the risk of Down syndrome (Trisomy 21 - T21), Edward's syndrome (T18) and Patau's syndrome (T13).
Second trimester test (15-20 weeks)
A triple test can be performed, which measures levels of free β-HCG, AFP and uE3. Interpretation of the results is shown below. This test can screen for T21 and T18, but serum tests cannot detect T13. Sometimes levels of Inhibin A are also measured, making it a quadruple test. Inhibin levels are high in cases of trisomy 21 and low in trisomy 18.
This combines the two first trimester tests with the triple test. However, results are not available until after 15 weeks.
Serum integrated test
This combines the first semester blood test with the triple test.
This is offered at 18-21 weeks of pregnancy and uses ultrasound images to study the fetus in detail. The organs are checked thoroughly for malformations, as are the spinal cord and limbs. Although many malformations of the fetus are identified, not all abnormalities are detected at this stage.
These tests can diagnose conditions prenatally but are more invasive than the screening tests and therefore carry a greater risk to the mother and child. They use samples of fetal tissue so can detect and diagnose chromosomal abnormalities.
There are two main tests used - Chorionic villus sampling and Amniocentesis.
In this test, a needle is inserted transabdominally under ultrasound guidance and 20-30ml amniotic fluid is removed.
This proceedure is performed between 15-20 weeks gestation. As it can provide a diagnosis, it is a very useful test and can inform parents before the birth so they have time to process the information, research the condition and make decisions about the optimal management for them. The results are more than 99% accurate.
However, there are some risks associated with this invasive proceedure. There is a 1% risk of miscarriage and a small chance of infection. The procedure is shown below.
In this test, cells are taken from the placenta which has identical genetic material to the fetus as they are both formed from the embryo. A speculum in inserted and a catheter is passed transvaginally through the cervix. Alternatively, a needle can be passed trans-abdominally to collect the cells. Both of these procedures are carried out under ultrasound guidance.
CVS is an alternative to amniocentesis and can be carried out earlier in the pregnancy, at 10-13 weeks gestation. This may be an advantage if termination of the pregnancy is a possibility, as it will be less unpleasant at this stage of the pregnancy. It may also give parents longer to prepare mentally and physically for the birth of a disabled child.
However, CVS carries higher risks than amniocentesis as it is done in early pregnancy. The risk of miscarriage is around 2%, and there is a small risk of infection. There is also a risk that rhesus sensitisation may occur during the proceedure.
CVS carried out earlier than 10 weeks gestation may cause limb deformities, so for this reason it is only available from 10 weeks onwards. The procedure is illustrated below.
This technique uses endoscopy to visualise the fetus and can identify structural abnormalities. It also allows fetal blood and tissue sampling for conditions such as sickle cell disease. It is usually performed at 18-20 weeks.
This test obtains fetal blood cells from the umbilical cord under ultrasound guidance. It can be used for chromosome analysis and to test for rhesus iso-immunisation.
X-ray images can be taken of the fetus at the time of the anomaly scan to look for skeletal dysplasia. MRI scanning can detect structural abnormalities and neurological problems.
There are a vast amount of ethical issues surrounding prenatal diagnosis. For example, by screening for these conditions at all some people believe we are labelling disabled children as undesirable and less valued members of society. Furthermore, as prenatal tests are constantly advancing in ability, it is already possible to determine the gender of an unborn child. There is a possible danger that parents may soon be able to 'design' their perfect child in terms of gender, physical ability and appearance.
More information about the importance of thorough explanation of the screening procedures and possible results can be found in the antenatal screening explanation article.
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