Special antenatal tests
During your antenatal care you may have a number of special screening and diagnostic tests to check for various complications or defects that can affect the baby.
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These tests can be reassuring if they rule out something you're worrying about, but they may reveal a problem that makes you question continuing with your pregnancy. This can put an enormous strain on parents-to-be. It's important to talk to your doctor beforehand about the risks of the tests and the implications of the results.
Older mothers
Your age is just one of several factors that can affect the outcome of your pregnancy. Your diet is much more important.
If your general health is good, you shouldn't be treated any differently from younger women during your pregnancy. Age is a factor, though, in certain fetal abnormalities, and an older mother may have a higher risk of maternal diabetes and placental insufficiency. You may be screened for these more frequently.
Maternal age seems to be a factor in Down's syndrome. The risk of having a baby without this condition rises as you get older, but isn't really significant until after 35 years of age. As most babies are born to women under 35 who don't have screening, there are more babies in the pre-35 age group than in the post-35 age group.
Screening tests
Most maternity units now have a number of tests available that screen for a variety of fetal abnormalities. These tests can't tell you for certain whether anything is wrong, but will give a probability. If a test shows that there may be a problem, a diagnostic test can confirm it or rule it out.
Nuchal scan
The risk of having a Down's syndrome baby can be assessed at around 11-14 weeks using a special ultrasound scan called a nuchal scan that measures the fluid at the back of the baby's neck. (“Nuchal” means neck.) All babies have some fluid, but a greater amount than normal may indicate a higher risk of chromosome defects such as Down's syndrome, especially in the older mother. If the scan shows there is a risk of Down's, amniocentesis can confirm the diagnosis. Many centres now carry out amniocentesis only after a nuchal scan has been done. Chorionic villus sampling (CVS) may be suggested as a way to detect problems.
Serum screening tests
This is also known as the Bart's triple test. A sample of the mother's blood is taken between 14 and 20 weeks to measure the levels of three substances - oestriol, human chorionic gonadotrophin, and alpha-fetoprotein. The results are assessed in relation to the mother's age to predict the chance of her baby suffering from Down's syndrome. If the chances seem high (more than one in 250), doctors may suggest you have amniocentesis. If you're not offered this test, you can ask to have it.
Combined test
Many units now offer the combined test either to all pregnant women or selectively to women over 35 years old as their risk of having a baby with Down's syndrome is statistically higher. With the combined test a nuchal scan is performed and a blood test is taken on the same day for the triple screening. The combined results are given to you about a week later, with an estimated overall risk of abnormalities.
Diagnostic tests
These tests are used to confirm abnormalities in the fetus, and are generally only used after screening tests or ultrasound scans have shown that you may be at a high risk. The main diagnostic tests are amniocentesis and CVS. Amniocentesis is the most common diagnostic test; although CVS can be carried out earlier, it is not available in all centres, and it carries a higher risk of miscarriage.
You and your partner will need to think very carefully about the implications of these tests and talk them through together and with your doctor. Ask for specialist counselling if you want it.
Amnio PCR
In many centres an amnio polymerase chain reaction (PCR), or rapid result test, is now offered. Although not quite as accurate as the cultured cells method, the results are available in 24-36 hours. The cultured cells result backs up the PCR findings in 99.8 per cent of cases.
Antenatal chorionic villus sampling (CVS)
Chorionic villi, finger-like outgrowths on the edge of the chorion, are genetically identical to the fetus. They develop earlier than amniotic fluid, so examining a sample of chorionic villi provides valuable information about your baby's genes and chromosomes before it's possible to carry out amniocentesis.
What can it reveal?
The most important group of mothers needing CVS are those at risk of having a Down's syndrome baby. An abnormality of haemoglobin, such as sickle-cell disease or thalassaemia, can also be diagnosed with CVS. Inborn errors of metabolism are fortunately rare, but if a family is afflicted, the incidence may be as high as one in four of their children. The basic defect is an enzyme deficiency, and direct enzyme analysis on the chorionic tissue gives a diagnosis within two days. Single gene disorders, such as cystic fibrosis, haemophilia, Huntington's chorea, and muscular dystrophy, can be detected with the use of CVS.
How is it done?
CVS is also carried out under ultrasound control, usually between ten and 12 weeks of pregnancy, before the amniotic sac completely fills the uterine cavity.
Two routes are used: the trans-cervical and the trans-abdominal. For the former, the cervix is first examined using a speculum. A plastic or metal catheter is then introduced through the cervical canal, across the uterine cavity, and then into the outside edge of the placenta. A small amount of chorionic villi tissue is then removed for analysis.
The second method of chorionic villus sampling is similar to that of amniocentesis, but with a sample being taken of the placental tissue rather than of the amniotic fluid. The risk of miscarriage following CVS is about one per cent higher than the spontaneous miscarriage rate. The advantage of CVS is that it gives an initial result within 24-48 hours, with full results in about a week. This is helpful if the risks are high and you don't want to have to wait until your pregnancy is at a more advanced stage for the results of amniocentesis.
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Posted 16.11.2010
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