The Tot Guide to prenatal genetic testing
Confused about the vast array of tests that are available to find out whether your unborn baby has a genetic disorder? Our simple guide explains all your options.
Pregnancy brings excitement and joy to parents-to-be, but it also comes with a dizzying number of appointments and tests. Prenatal genetic tests can be particularly confusing and technical, so we’ve created a clear and concise guide to help you understand your options.
What are genetic disorders?
A genetic disorder is caused by an abnormality in a person’s genes or chromosomes. Aneuploidy occurs when there is an extra chromosome (such as with Down syndrome) or a missing chromosome (as in Turner syndrome). Inherited disorders, such as cystic fibrosis, sickle cell disease and Tay-Sachs disease, are caused by genetic mutations.
What are the different types of prenatal genetic tests?
There are several ways to test for genetic disorders before or during pregnancy. Prenatal genetic tests are optional, but they can help you make important decisions regarding your pregnancy and prepare for the arrival of your baby.
Carrier screening tests
These tests use a blood sample or cheek swab to determine whether you and your partner carry the genes for certain inherited disorders. They’re most beneficial before you fall pregnant, but they can also be done during pregnancy.
People from certain ethnic backgrounds – including Eastern European Jewish, African, Mediterranean, Southeast Asian and non-Hispanic white – are more likely to be carriers for certain conditions, so ask your doctor about getting tested if you fall into one of these categories.
Prenatal genetic screening tests
Your doctor will probably recommend one or several of the following tests during your pregnancy.
This combines a blood test and an ultrasound exam known as a nuchal translucency screening to assess the chance that your baby has Down syndrome, trisomy 18, a neural tube defect such as spina bifida, or another genetic disorder. It’s performed between 10 and 13 weeks of pregnancy and can be followed up with more tests in the second trimester if you want more accurate results.
Cell-free DNA testing:
Also known as non-invasive prenatal testing (NIPT), this simple blood test can be performed as early as 10 weeks and screens for Down syndrome, trisomy 13, trisomy 18 and problems with the number of sex chromosomes. It can also reveal your baby’s gender earlier than other tests. If your results are positive, your doctor will most likely recommend a follow-up diagnostic tests.
The quad screen (also known as the quadruple marker test) is performed between 15 and 22 weeks of pregnancy and tests the levels of four different substances in your blood to evaluate the probability that you’re carrying a child with Down syndrome, trisomy 18 or a neural tube defect. Because it isn’t as accurate as first-trimester screening, it’s only recommended for women who didn’t do any genetic testing in the first trimester.
Between 18 and 22 weeks, you’ll also undergo an ultrasound exam to look for major defects in the brain, spine, kidney, heart, limbs and facial features.
Combined first- and second-trimester screening:
There are several ways in which first- and second-trimester tests can be combined. The most common is an integrated screening, which involves repeating the first-trimester screening in your second trimester or combining the results of your first-trimester screening and your quad screen. It takes longer to get your results, but they’re more accurate.
Prenatal genetic diagnostic tests
These tests check for birth defects by analyzing cells obtained from the fetus or placenta and they’re 99 percent accurate. Your doctor may recommend a diagnostic test to confirm the results of a positive screening test, but it’s up to you whether you go ahead with it.
Performed between 15 and 20 weeks, an amniocentesis involves inserting a long needle though your belly and into the amniotic sac to withdraw some amniotic fluid. The cells and proteins in the fluid are analyzed for disorders such as Down syndrome, trisomy 13, trisomy 18 and neural tube defects. There’s a very slight chance of miscarriage with amniocentesis.
Chorionic villus sampling (CVS):
A sample of tissue is taken from your placenta by inserting a needle in your belly or a tube in your cervix. The tissue is analyzed for Down syndrome, trisomy 13 and trisomy 18 and other genetic disorders. CVS can be performed earlier than amniocentesis (between 10 and 13 weeks), but the risk of miscarriage is slightly higher.
Preimplantation genetic diagnosis:
Couples who are undergoing in vitro fertilization (IVF) and who are at increased risk of having a baby with a genetic disorder can have their embryos tested for abnormalities before implantation.
Should you do genetic testing?
Whether or not you choose to undergo prenatal screening or diagnostic tests is completely up to you. It’s a good idea to discuss your options and preferences with your doctor and partner before you fall pregnant or early on in your pregnancy to ensure that you’re informed and in agreement.
Some couples prefer not to find out that their child might have a genetic disorder before they’re born, while others want to know so they can prepare for their baby’s arrival. In some cases, parents might choose to terminate the pregnancy. The only right decision is the decision that feels right for your family.
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