Genetic testing – what to consider

Posted on 18 April 2013

Dr Linnie Muller has managed the Prenatal Genetics & Fetal Medicine Centre at Mediclinic Panorama for the last 20 years. She is recognised as one of South Africa’s leading experts in fetal medicine and fetal echocardiography. She explains when you should consider genetic testing if you’re expecting, as well as tests you may need to have during your pregnancy.

I’m 12 weeks pregnant. When should I consider genetic testing as an expectant mom? What are the factors that would put me at an increased risk of having a baby with a genetic abnormality or disorder?

These are the factors you should consider:
•    Your history: If you have a history of a previous pregnancy or child with a genetic abnormality.

•    Your family history: If you or your partner carries a chromosomal or genetic disorder, or a family history that puts your child at increased risk for genetic problems.

•    Your age: Anyone can have a baby with a chromosomal abnormality, but the risk increases with maternal age. For example, your likelihood of carrying a baby with Down syndrome ranges from about 1 in 1 000 at age 25 to 1 in 70 at age 40.

If you’ve already had some screening tests done and these are your results, here’s when you should consider seeing a genetic counsellor:
•    A first trimester screening or nuchal fold scan: This indicated your baby is at a higher risk for Down’s syndrome or another chromosomal problem.

•    Ultrasound results: An early ultrasound that your baby has structural defects associated with a chromosomal problem.

•    Carrier screening results: You and your partner are both carriers of a recessive genetic disorder, for example cystic fibrosis or sickle cell disease.

If I decide to go for screening, how do I decide what’s right for me?
Because every pregnancy has a small risk of carrying a fetus with an abnormality, women of all ages should be offered first and second trimester screening, and diagnostic testing options. But ultimately, whether or not to test is a personal decision.

Many women opt for screening and then make a decision about diagnostic testing based on the initial results, while other women opt for diagnostic testing right away. Some decide to have no screening or testing.

If you opt for screening first, you can then choose – with the help of your practitioner or genetic counsellor – whether your results indicate a high enough risk that you want to have Chorionic Villus Sampling (CVS) or an amniocentesis to determine whether a problem exists. You’ll need to weigh your need to know about your baby’s condition against the small chance that diagnostic testing could cause a miscarriage. Just remember, there’s no right decision. Individual parents have different feelings about what risks are acceptable and may arrive at different conclusions when facing the same set of circumstances.

If I choose diagnostic testing, how do I decide between CVS and amnio?
Both tests can tell you whether your baby has a chromosomal problem or certain genetic disorders. CVS is done earlier in pregnancy (usually between 11-13 weeks), so you can find out sooner about chromosomal defects. It is still too early to exclude major structural defects. There is also about a one percent risk of chromosomal mosaicism. (Mosaicism means that some of the cells in the culture show one chromosome arrangement and some of the cells in the culture have a different chromosome arrangement). Keep in mind; if your CVS is inconclusive, you’ll be required to have a follow-up amniocentesis to determine the correct answer. The amniocentesis is done between 15-16 weeks, and can also rule out having a baby with a neural tube defect, and does not carry the risk of mosaicism. The foetus is also slightly bigger so other defects in the brain or heart can also be checked before doing the invasive procedure.

Before making a decision, you’ll want to discuss all of these issues with your partner, your health practitioner, and possibly a genetic counsellor.

For more information, visit Fetal Medicine.

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The information provided in this article was correct at the time of publishing. At Mediclinic we endeavour to provide our patients and readers with accurate and reliable information, which is why we continually review and update our content. However, due to the dynamic nature of clinical information and medicine, some information may from time to time become outdated prior to revision.