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Women are routinely offered a variety of genetic screening tests during their first three months of pregnancy to evaluate the risk for genetic disorders in their unborn baby. The first trimester screening tests are usually done between the 10th and 13th week of pregnancy. They involve measuring the level of certain substances in the mother's blood and obtaining an ultrasound. Genetic screening tests can be done during the second trimester as well.
Information from these screening tests, along with other risk factors such as a woman's age and a couple's ethnic background and family history of genetic disorders, are used to help calculate the odds that the fetus might be born with genetic disorders, such as Down syndrome, cystic fibrosis, Tay-Sachs disease, or sickle cell anemia.
Birth defects affect 1 in 33 babies born in the United States each year, according to the Centers for Disease Control and Prevention (CDC). The CDC also said that birth defects can occur at any point during pregnancy, but most of them take place during the first trimester, when the baby's organs are forming.
Screening tests can help tell parents-to-be whether the fetus may be at high or low risk of having a chromosomal abnormality, but the only way to actually know if the baby will be born with a problem in its genetic makeup is by doing diagnostic testing, according to the National Institutes of Health (NIH).
Whether or not a woman decides to undergo genetic screening is her own choice, as positive results could produce anxiety and conflicting emotions.
Making a choice
Genetic screening is offered to all pregnant women, and it's usually discussed during the first prenatal visit, said Dr. Andrea Greiner, a maternal and fetal medicine specialist at the University of Iowa Hospitals and Clinics. "It's optional, but not required."
In trying to decide whether or not to undergo prenatal genetic screening, Greiner recommended that moms-to-be ask themselves if they want to know if their babies risk for certain chromosomal abnormalities or genetic defects is increased, and if it is, then how will this information affect them.
"Every woman wants to believe that her pregnancy is normal and uncomplicated," Greiner said. If a pregnant woman chooses to have genetic screening, there is a possibility the results could come back abnormal, she added, explaining that a lot of women don't go through that thought process before getting screened but need to.
Greiner said that one common misconception about genetic screening and testing that some pregnant women have is the only reason to do them is if they were going to have an abortion because of a positive result. But she said that's not the case.
"Most women do prenatal genetic testing to know what the risk is before the baby is born," Greiner said. They would rather know the information during pregnancy than at birth to make plans or gain further knowledge, she explained.
Practical implications of testing
Greiner explained the key differences between genetic screening and diagnostic testing, as well as how and when these tests are done and their practical implications:
Screening vs. diagnostic testing
Genetic screening and genetic testing are not the same things. Genetic screening is measuring a level of risk for genetic diseases in the fetus, Greiner said.
Screening tests assess the degree of risk, or chance, that the fetus may potentially have certain common birth defects, but they cannot tell with certainty if the baby actually has the problem, according to The American College of Obstetricians and Gynecologists.
There are no dangers to the mother or the fetus from these screening tests, but positive screening results are not diagnostic.
A new screening test done at 10 weeks of pregnancy, called cell-free fetal DNA testing, uses the mother's blood to detect Down syndrome. Not covered by all health insurances, cell-free fetal DNA testing should only be used by women who are at high risk for chromosomal abnormalities, Greiner said.
"The science is still very new and patients should use some caution," she explained, but Greiner said it's important for women to know that cell-free fetal DNA is not a replacement for diagnostic testing if it shows a positive result.
Pregnant women are recommended to follow-up a positive screening result with genetic counseling and undergoing one of two invasive diagnostic tests, which have greater accuracy and reliability than genetic screening alone.
Diagnostic tests can actually detect many genetic conditions caused by chromosome abnormalities. They usually can tell prospective parents whether or not their fetus has a particular genetic problem.
Preconception genetic screening vs. prenatal genetic screening
During pre-conception genetic screening, carrier tests can be done before pregnancy to determine whether the mother or father carry a gene for genetic disorders that might run in families, such as cystic fibrosis and sickle cell anemia, and could be passed on when the couple conceives.
Genetic counselors are trained health professionals, who can help explain the meaning of screening results and interpret the risks of an inherited genetic disorder.
In prenatal genetic screening, a pregnant mother receives a blood test to determine a level of risk for genetic conditions in the fetus.
Pros and cons of genetic screening
When women need help weighing the pros and cons of genetic screening, Greiner said she tells them to consider several questions: First, she asks them, "Would they want to know prior to their baby's birth if the child was at an increased risk for common chromosomal abnormalities?"
Second, Greiner suggested women think about "What would a positive screening test result mean to them?"
One of the benefits of undergoing genetic screening is that a pregnant woman obtains information about possible chromosomal abnormalities in the fetus.
But the disadvantages are that if the screening yields positive results, then a woman and her partner will have to deal with this new information and choose whether or not to act on it. They may follow-up by seeking out genetic counseling and undergoing prenatal diagnostic tests to more accurately detect whether the fetus actually has a genetic condition.
Genetic screenings can give false positive results, meaning they can be wrong and lead expectant parents to believe their unborn babies might have genetic abnormalities when they do not. There's also a chance the screening will not pick up a chromosomal abnormality when there is one.
That's why pregnant women should not be making decisions about terminating a pregnancy based on a positive screening result alone without getting an invasive diagnostic test to confirm or rule out a diagnosis.
Pros and cons of genetic diagnostic testing
Greiner said that some of her patients ask her why they should do prenatal genetic testing because if there is a positive result, the genetic problem detected in the baby can't be changed, fixed or treated.
Her answer to that question is that no one likes a surprise. Knowing this information in advance can help expectant parents plan and prepare themselves as well as their home for a baby who may have special needs.
Greiner said women need to ask themselves whether a positive diagnostic testing result would cause them anxiety if they choose to continue the pregnancy, or if these genetic findings may provide reassurance if they decide to terminate the pregnancy as a result of the diagnosis.
Two common types of diagnostic testing
Both diagnostic tests are invasive and require that cells be extracted from the fetus and analyzed under a microscope. This allows geneticists to determine whether the fetus has too few or too many chromosomes present, or if the chromosomes are damaged and could result in a genetic problem.
Chorionic Villus Sampling (CVS)
Done during the first trimester of pregnancy usually at 10 to 12 weeks, this diagnostic test involves taking a small sample of cells from the placenta. Placental tissue contains the same genetic material as the fetus and can be checked for chromosomal abnormalities and other genetic disorders. CVS cannot identify neural tube defects, such as spina bifida, which can be detected by amniocentesis.
How it's done: Depending upon where the placenta is located and using ultrasound for guidance, a small tube is inserted through either the mother's abdomen or her vagina and a small tissue sample is withdrawn from the placenta.
Possible risks: CVS has a slightly higher risk of miscarriage than amniocentesis. CVS has a 1 percent risk of miscarriage, according to the Mayo Clinic.
"Amniocentesis is considered the gold standard for prenatal genetic testing," Greiner said.
How it's done: A long, thin needle is inserted into the mother's abdomento extract a sample of the amniotic fluid surrounding the fetus. The procedure is usually done between the 15th and 20th week of pregnancy, and the amniotic fluid contains cells from the fetus with genetic information about the unborn child.
Possible risks: Amniocentesis carries a lower risk of miscarriage than CVS, about 1 in 400, Greiner said.
- Learn more about chorionic villus sampling from the Mayo Clinic.
- NIH: What tests might I need during pregnancy?
- CDC: Facts About Birth Defects