Approximately 2% of babies are born with some form of genetic irregularity that causes a degree of physical, developmental and or intellectual disability.
There are pregnancy tests that can detect some congenital irregularities but there is no test that can guarantee that your baby is perfect physically and mentally.
Some parents opt for genetic testing because they want to be reassured that their baby is genetically normal. Others want the information these tests can provide so that they can either, prepare for a baby who has a disability or, so that they can decide whether or not to terminate the pregnancy. Some parents opt not to undergo any form of genetic testing, preferring to wait till their baby is born to discover whether or not s/he has a genetic abnormality.
Counseling should be available to both parents before any genetic testing is undertaken and afterwards if the test result is positive for some abnormality.
Before you make a decision about whether or not to have a test and what you might do in response to the information gained from the test you need to know whether the test is a screening test or a diagnostic test.
Screening tests assess whether your baby is at high or low risk of having a particular condition, whereas diagnostic tests can identify babies who actually have a particular condition (although mistakes in interpretation and diagnosis are sometimes made).
Screening tests will result in a number of false positive and false negative results. A false positive is a result that indicates that your baby may have a genetic abnormality when this is not the case. A false negative result indicates that it is unlikely that your baby has a particular genetic abnormality when in fact s/he does.
Nuchal Translucency Screening(NT) – A screening test
This scan is performed when a woman is between 11 weeks and 13 weeks 6 days pregnant and assesses the risk of the baby having Down Syndrome, neural tube defects like spina bifida and some other cardiac (heart) abnormalities. Increased thickness in the fluid-filled nuchal space at the base of the baby’s neck increases the probability that the baby has Down Syndrome(Trisomy 21) or some other chromosomal (Trisomy 18 – Edwards Syndrome; Trisomy 13 – Patau Syndrome; Triploidy, Neural Tube defects) and/or some cardiac abnormalities.
On it’s own the NT scan is believed to detect about 80% of babies who have conditions associated with increased nuchal thickness but also results in false positive results for 3% of babies scanned. The accuracy of NT scanning is increased if it is combined with two blood tests to measure the levels of “markers” (pregnancy proteins and hormones) in the mother’s blood. The first blood test should be taken sometime from week 9 of pregnancy up to about one week before the NT Scan. The second blood test, called a maternal serum screening(MSS) should be taken between 14 and 18 weeks of pregnancy.
The results from the blood tests are combined with the mother’s age and the NT Scan results to give individual risk assessment. If you have both blood tests and the NT scan, the results were predicted to be approximately 92% accurate although a recent NZ audit of the results for this First trimester screening has shown that the combined NT and blood test in fact only detect about 78% of babies with any of the conditions covered by this screening test.
Chorionic Villus Sampling (CVS) – a diagnostic test
CVS is a procedure which takes a sample of chorionic villus (part of the outer membrane around the developing embryo which will later form the fetal side of the placenta) and tests this tissue for genetic abnormalities. (CVS cannot detect spina bifida.) This sample is taken by inserting a needle into the uterus through the abdomen or through the vagina using ultrasound for guidance. (Research suggests that the transabdominal CVS results in a lower failure rate, less bleeding and fewer miscarriages.) This test is usually performed when the mother is about 12 weeks pregnant and preliminary results are usually available after about 7 – 10 days, full results are usually available about 3 weeks after the procedure.
This test identifies over 99% of babies with genetic abnormalities but carries a 0.5 – 1% risk of miscarriage. Other risks include maternal infection and bleeding and a small rate of false-positive diagnoses. Some researchers are also concerned that CVS may rarely cause face or limb abnormalities, especially if performed before 10 weeks gestation.
Amniocentesis – A diagnostic test
This test is used to detect chromosomal disorders (e.g. Down syndrome, Turner Syndrome), genetic disorders (if there is a family history of e.g. cystic fibrosis, Tay-Sachs disease) and neural tube defects e.g. spina bifida.
Amniocentesis involves taking a sample of amniotic fluid and analysing the fetal cells in this sample for chromosomal or biochemical abnormalities or damage. This test cannot be performed until the woman is about 16 weeks pregnant because there is insufficient amniotic fluid before this time.
The mother is often given muscle relaxant medication before the procedure is performed. The procedure is guided by continuous ultrasound. A pocket of amniotic fluid that is a safe distance from both the baby and the placenta is located (this can take 10 – 20 minutes); the specialist inserts a hollow needle through her abdominal wall into the pool of amniotic fluid and withdraws a small amount of amniotic fluid.
This fluid sample is sent to the laboratory for analysis. Some results are available after a few days, but the fetal cells have to be separated from the amniotic fluid and cultured (grown) in the laboratory for a week or two before they can be tested for chromosomal and genetic abnormalities. The mother/parents usually receive full results about 15 days after the test.
Amniocentesis is about 99% accurate for the conditions it tests for. Approximately 1:200 women who has an amniocentesis will be found to be carrying a baby with a genetic abnormality, however about 1:200-300 women will miscarry as a result of this procedure
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