While most babies are born healthy, approximately 3-5% of babies are born with a birth defect. Some of these defects are caused by or associated with genetic disorders or syndromes. Some pregnancies are at increased risk of these birth defects or genetic disorders. These include mothers over the age of 35 at the time of delivery, a personal or family history of a birth defect or genetic disorder, and various other situations.
Genetic disorders are caused by changes in a person’s genes or chromosomes. These changes are known as “mutations”. Aneuploidy is a condition where the chromosomes are off in number. If a chromosome is missing, it is known as monosomy; if there is an extra chromosome, it is referred to as trisomy. Trisomy 21 (or Downs Syndrome) is one of the most known aneuploidy conditions.
Historically, the only way to determine if a baby was at risk of genetic abnormalities was based on certain findings (known as “markers”) seen on the 20-week anatomy ultrasound. By this point in time, parents who were given bad news were faced with limited options as state termination laws generally prohibit ending a pregnancy beyond 20-24 weeks. Modern medicine recognized this problem and worked to develop testing methods which were more accurate and performed sooner, affording parents more time to decide what course of action was best to take given their individual situation. Various types of genetic testing have been developed including both screening tests and diagnostic tests. These can identify genetic abnormalities only, not structural abnormalities.
Prenatal screening tests can tell you the chances that your baby has an aneuploidy; it does not confirm a diagnosis and it may include false-positive (a positive result when the baby is actually normal) or false-negative (a negative result when the baby is actually affected) results. It includes lab tests performed during the 1st and/or 2nd trimester (depending on which test is used) and ultrasounds performed at 12 weeks and 20 weeks. The most accurate test is called cell-free DNA testing. It is a non-invasive prenatal test where genetic analysis is performed on some of the baby’s cells which are floating in mom’s blood. This test was designed for high risk patients but may be offered to all. There may be a cost associated with these tests. If positive results are encountered, patients will then be referred to have diagnostic testing performed to confirm the results.
Prenatal diagnostic tests can tell you whether or not your baby actually has a genetic abnormality. These tests are done on cells from the placenta (chorionic villus sampling) or the amniotic fluid (amniocentesis).
Chorionic villus sampling (CVS) is a diagnostic test performed during the first trimester (between 10-13 weeks of pregnancy) to determine whether or not a baby has a normal number of chromosomes. CVS is performed by inserting a core needle through a mother’s abdomen, into the uterus, and removing a small piece of the placenta. This tissue is analyzed in a laboratory and genetic testing is performed. The risk of miscarriage after CVS is approximately 1 in 250.
Amniocentesis is a diagnostic test to determine whether or not a baby has a normal number of chromosomes. It is typically performed during the second trimester (between 15-18 weeks of pregnancy) but can be performed all the way up to the time of birth. An amniocentesis is performed by inserting a very thin needle through a mother’s abdomen, into the uterus, and withdrawing a small amount of amniotic fluid. Floating in that fluid are some of the baby’s cells that have naturally fallen off. These cells are then used by the laboratory for genetic testing. The risk of miscarriage after an amniocentesis is 1 in 1000.
While prenatal genetic testing is concerned with the genes of the baby, carrier screening analyzes the genes of the mother and/or father to determine whether or not they have any unknown genetic abnormalities which can pass to their child and cause a disorder. Normal humans have two copies of every gene. For certain genetic disorders to occur, a person with that disorder must have two damaged copies of the same gene. As long as a person has at least one working copy of the gene, he or she is normally healthy and usually has no signs or symptoms of the genetic disorder. People who have one working copy and one damaged copy are called “carriers”. It is estimated that we all “carry” at least 8-10 of these damaged genes in our DNA. While having only one damaged gene doesn’t matter to us, if we have children with someone who also carries a damaged copy of the same gene, these children are at increased risk for having the genetic disorder.
Carrier screening is offered to all people who are pregnant or planning on becoming pregnant. Because people of certain ethnicities are more likely to be carriers of certain disorders, most carrier screening is offered based on the ethnicity of you and the father of the baby. Some of the common genetic disorders screened for are cystic fibrosis (CF), fragile X, spinal muscular atrophy, (SMA), sickle cell anemia, and Tay Sachs disease. There are often no cures or treatments for these disorders. A baby can only be affected if both parents are carriers, and even then it is only a 25% risk the child will be affected.
Carrier screening is done through a blood test on the female partner first. If the test result shows that the female partner is not a carrier, then no additional testing is needed. If the test result is positive, then the father of the baby is tested. The results of a carrier screen do not change from pregnancy to pregnancy and therefore mothers only need to be tested once time. (This is in contrast to genetic testing which is baby-specific to each pregnancy).
It is a personal decision on whether or not to have genetic testing or carrier screening. Your individual beliefs and values are important factors in this decision. Some women make these decisions based on whether or not the results would change their plans to continue the pregnancy. Others choose to have testing while planning to continue their pregnancy no matter what, because they want time to possibly prepare for having a baby with a genetic disorder. Some want testing as reassurance on the likely positive outcome. Other parents do not want to know any information before the child is born and decide not to have testing at all. Financial considerations and support resources may also influence a family’s decision on whether or not to have testing performed. There is no right or wrong answer. Certain tests can only be done at certain times in pregnancy and tests performed earlier allow parents more time to make decisions if a result is positive. If ending the pregnancy would be considered, it is safest to do so within the first 13 weeks of pregnancy.
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