What Moms-to-Be Should Know About Prenatal Genetic Testing
When a woman finds out she is pregnant, doctors appointments and a barrage of information comes quickly. This includes early decisions about prenatal genetic testing.
The optional tests, which can help detect the risk of abnormalities, can supply important information before a baby’s birth. Most genetic testing comes in the form of a simple blood draw which can provided a large amount of information about the baby.
Although the results can help families make informed decisions, the tests aren’t perfect. Blood-based screening tests cannot always say with certainty that a baby is affected, a deficit that can be difficult to handle during pregnancy. This is why it is a good idea to talk about any type of genetic testing before a baby is conceived.
Some of those discussions are easier to have outside the context of pregnancy. But a lot of couples should consider talking about what they would do if the results are positive.
There are two blood-based methods to screen for chromosome abnormalities in pregnancy. These options includes:
- First-trimester screening: This test has two parts: bloodwork looking at hormones in a mother’s blood and an ultrasound. The ultrasound is used to measure the back of the baby’s neck. extra fluid on the neck can be a sign of Down’s syndrome, heart defects or other complications.
This information is used to provide an overall risk assessment. The test will detect about 93 percent of Down’s syndrome cases with a 5 percent false positive rate. It also screens for other chromosome abnormalities and can sometimes detect pregnancies at high risk for complications such as poor growth later in pregnancy.
- Cell-free DNA screening: DNA fragments from the placenta that are in a pregnant mother’s blood can be used to try to identify pregnancies at higher risk for chromosome abnormalities.
This test is the most sensitive screening for Down’s syndrome, trisomy 13 and trisomy 18 (genetic disorders that cause birth defects) and provides some information about the baby’s sex chromosomes. It has a lower false positive rate than the first-trimester screening, but does not provide other information about a baby’s health.
Each screening option may be administered as early as 10 weeks into a pregnancy.
Diagnostic testing in pregnancy
Diagnostic tests provide more accuracy as well as information about conditions not included in screening. This can be particularly informative when certain birth defects are detected on an ultrasound. Some women may choose to have these invasive tests without screening. Others could have an abnormal screen and then proceed to a diagnostic test.
Diagnostic testing options include:
- Chorionic villus sampling: Placenta tissue is extracted via the cervix (a similar approach to a Pap smear) or by using a needle inserted through the mother’s abdomen into the uterus. It is performed between the 10th and 13th weeks of pregnancy. Parents who want a confirmation sooner often choose this option.
- Amniocentesis: In this test, a needle is inserted through the abdomen to extract fluid surrounding the baby (the needle doesn’t touch the baby). The amniotic fluid contains fetal cells that can be used for diagnostic testing. It is performed 15 weeks into the pregnancy or later.
There is a risk of miscarriage with these procedures, but most women who undergo the procedures have no complications.
Talk to your doctor if you have more questions about prenatal care.Tags: genetic testing, pregnancy, pregnancy testing, prenatal testing