PREGNANCY: SPECIAL TESTS
Amniocentesis. Similar principles apply to this test which is performed between about fourteen and sixteen weeks gestation. Ultrasound is used to identify the foetus and placenta, and the fluid surrounding the foetus (amniotic fluid). A fine needle is passed through the woman’s abdominal wall (which has been numbed with local anaesthetic) to the uterus and about 15 millilitres of amniotic fluid is removed.
Tests on the fluid can indicate the sex of the foetus, chromosomal abnormalities and whether the foetus has any degree of spina bifida. The results of the chromosome tests take longer than the spina bifida tests, but all results should be available by the end of three to four weeks.
The risk of miscarriage is smaller with this test than with CVS, and is probably around one in 200, if that. Amniocentesis also tests for two types of condition; CVS only tests for one. But amniocentesis is done later in the pregnancy, so that if a woman decides on a termination, it is not a simple D and C procedure, but more like a labour. The generally high standard of antenatal ultrasound these days means that most routine scans will pick up the majority of degrees of spina bifida.
Triple lest. This test (which is not routinely available in Australia at the time of publication) is a blood test which can be performed in the first trimester to screen for Down’s syndrome. It measures the level of HCG, oestriol (a type of oestrogen) and alpha-fetoprotein (a protein) in the woman’s blood. A certain level of each component may indicate an increased risk of Down’s syndrome in the foetus, and the woman can then choose to have further testing, such as CVS or amniocentesis. The triple test has a significant false-positive and false-negative rate, meaning it may be positive when the foetus does not have Down’s syndrome, and may also miss some cases. It is therefore not relied upon on its own, but can be used to help identify women who, despite having no obvious risk factors, may have a Down’s syndrome-affected foetus.
The test is used routinely as a screening test in some parts of the United Kingdom, but its place in routine antenatal care in Australia is not yet defined.
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