Diagnostic Tests During Pregnancy
Special diagnostic tests can also be carried out to identify problems that may affect your baby’s development and future health by detecting abnormalities or complications during your pregnancy.
They are offered to pregnant women in certain circumstances (such as where there is a family history of defects, or due to maternal age), and in situations where early blood checks or screening tests produce positive (high-risk) results.
While it may be difficult to decide if you want to go ahead with a particular diagnostic test, especially since some carry a risk of miscarriage during pregnancy, remember that it may offer you reassurance and give you the option early on in your pregnancy to do what you feel is best in the given circumstances. All these issues should be discussed in detail with your healthcare provider or genetic counsellor.
What types of diagnostic tests are available?
A number of tests, which are all carried out under the guidance of ultrasound techniques, are currently used to detect any abnormalities of your baby, although these are invasive and carry some degree of risk of miscarriage.
These tests are only available at certain hospitals in Ireland and can be discussed with your health professional.
This test is available to pregnant women (usually at weeks 14–18) who have been found to be at increased risk of having a baby with Down’s syndrome using screening tests (nuchal translucency or blood tests). The policy may vary in different areas.
In general, this test is offered to women where the risk of having a baby with Down’s syndrome is more than 1 in 250–300. Women with risks that are lower than this, but who remain anxious about Down’s syndrome, can also have amniocentesis, but will need to discuss this with their obstetrician or genetic counsellor.
Amniocentesis is also used for cases where a woman or her family has a history of an inherited disorder. Additionally, amniocentesis can reveal the sex of your baby, although it should not be used for this purpose unless there is a family history of serious gender-linked problems such as muscular dystrophy or haemophilia.
An amniocentesis test can also detect rhesus incompatibility and metabolic or other chromosomal disorders. The procedure involves a thin needle being inserted into your abdominal wall in order to aspirate a sample of the amniotic fluid, which is found in the sac of water that surrounds your baby.
In the laboratory, the amniotic fluid will be cultured to encourage the fetal cells to multiply (these are mostly the cells from the baby’s skin and urinary tract). At a certain stage of this ‘culturing’, the chromosomes show and can be analysed for obvious defects. Results of the test, however, can take up to 2–3 weeks to come through, and note also that the procedure itself is associated with a small risk (1%) of miscarriage so bear this in mind when deciding whether to go ahead with it or not.
There is also a 1-in-200 chance that the cells will not grow in culture, so you will need to decide whether to have a repeat test or not. Some centres now offer a rapid DNA test for Down’s syndrome, though you may have to pay for this. The DNA test gives a result for Down’s syndrome and some of the other main chromosomal problems in almost all cases. This works almost all of the time, so culture failure would not be a problem.
CVS (chorionic villus sampling)
The CVS test is used to detect defects associated with chromosomes (Down’s syndrome) or genes (including sickle-cell disease, thalassaemia, haemophilia, cystic fibrosis, and muscular dystrophy) and is offered to pregnant women after week 10 of their pregnancy, up until week 14.
The technique involves a fine needle being passed through the abdomen, or a fine tube through the vagina and cervix, into the womb to collect a small piece of the placental tissue, known as the chorionic tissue/villi. Results are usually available within a few days to 3 weeks. Again, however, the test carries a small risk, 1-2%, of miscarriage. There is also a 1% chance of a chromosome result called mosaicism, which is often only found in the villi or placenta and not in the baby; if so you may need to undergo further testing by amniocentesis.
Cordiocentesis/Umbilical vein sampling/Foetal blood sampling
This procedure is usually carried out only after 18 weeks of pregnancy to examine your baby’s blood for various reasons, such as where there is a need for rapid confirmation of chromosomal problems and where an intrauterine blood transfusion may be necessary for the baby, as in cases of Rhesus incompatibility or for foetal anaemia.
Foetal blood sampling also allows the detection of German measles, toxoplasmosis and oxygen shortage in babies that are growing much too slowly. With the exception of giving a blood transfusion to anaemic babies, it is rarely performed nowadays, as much of this information can now be obtained by DNA tests on amniotic fluid or by Doppler studies of the baby’s circulation. It involves a hollow needle being inserted into the umbilical cord to collect a small amount of blood from the baby. The risk of the test to your baby is 1-2%.
This method is only used in rare circumstances where a closer examination of your baby is necessary for health reasons. It involves a mini telescope (a small viewing device) being passed into the womb through a small incision. This allows examination of your baby in your womb and for a blood or tissue sample to be taken, or surgical correction of a foetal abnormality to be undertaken, if required.