Finding out that your child has a diagnosis of Trisomy 18 during your pregnancy is life-changing. You face an uncertain future and have many, many questions.
It is very likely that before learning of your child’s diagnosis, you were unfamiliar with Trisomy 18. While considered rare medically, Trisomy 18 is a life-threatening genetic disorder that impacts about 1 out of every 2000 pregnancies in the U.S.
Here’s what you need to know about how Trisomy 18 is diagnosed and what prenatal testing options are available.
How Is Trisomy 18 Diagnosed?
Most cases of Trisomy 18 are diagnosed prenatally in the United States. Regardless of whether the diagnosis is made prenatally or postnatally (after birth), the process is the same. A sample of the baby’s DNA is extracted from a blood sample or other bodily cells or tissue and is cultured to examine a picture of the chromosomes called a karyotype.
A karyotype is simply a picture of a person’s chromosomes. In order to get this picture, the chromosomes are isolated, stained, and examined under the microscope. Most often, this is done using the chromosomes in the white blood cells. A picture of the chromosomes is taken through the microscope. A visible extra 18th chromosome confirms a Trisomy 18 diagnosis.
A lot of prenatal testing is available that may indicate Trisomy 18. It is important to understand that there are two types of testing: screening and diagnostic.
What Is a Screening Test?
These tests indicate a risk or likelihood that Trisomy 18 is present. These tests take the results of everyone who has had the same testing, and they compare your specific results with that group. Then they use statistics to identify the odds that it is present in your child, based upon the number of times others with the same test results have had children with Trisomy 18 in the past.
This is much the same way that weather is forecast, by saying there is a 20% chance of rain because 20% of the time when the conditions were the same, it has rained. Just as the weather forecast is not completely accurate, screening tests are not a diagnosis but only an indication that the risk is higher than normal.
The following are screening tests, which cannot diagnose Trisomy 18:
AFP (also known as triple screen, quad screen, maternal serum screening)
The quad screen — also known as the quadruple marker test, the second-trimester screen or simply the quad test — is a prenatal test that measures levels of four substances in pregnant women’s blood:
- Alpha-fetoprotein (AFP), a protein made by the developing baby
- Human chorionic gonadotropin (HCG), a hormone made by the placenta
- Estriol, a hormone made by the placenta and the baby’s liver
- Inhibin A, another hormone made by the placenta
Ideally, the quad screen is done between weeks 15 and 18 of pregnancy — during the second trimester. However, the procedure can be done up to week 22.
The quad screen is used to evaluate whether your pregnancy has an increased chance of being affected by certain conditions, such as Trisomy 18. If your risk is low, the quad screen can offer reassurance that there is a decreased chance for Trisomy 18 or other conditions.
If the quad screen indicates an increased chance of one of these conditions, you might consider diagnostic testing to determine whether your baby has Trisomy 18.
Ultrasound (standard, level II, level III, 3D)
Diagnostic ultrasound, also called sonography or diagnostic medical sonography, is an imaging method that uses high-frequency sound waves to produce images of structures within your body. Ultrasound is used during pregnancy to check the baby’s development, the presence of multiple pregnancies and to screen for conditions such as Trisomy 18.
If fetal abnormalities are detected, you may be offered further tests to confirm the diagnosis, such as amniocentesis and chorionic villus sampling.
What Is a diagnostic test?
These tests check actual cells and can determine if Trisomy 18 is actually present. This is a diagnosis since the condition has actually been found in the cells.
The following are diagnostic tests, which CAN diagnose Trisomy 18:
CVS (Chorionic Villi Sampling)
Chorionic villus sampling (CVS) is a prenatal test in which a sample of chorionic villi is removed from the placenta for testing. The sample can be taken through the cervix (transcervical) or the abdominal wall (transabdominal).
During pregnancy, the placenta provides oxygen and nutrients to the growing baby and removes waste products from the baby’s blood. The chorionic villi are wispy projections of placental tissue that share the baby’s genetic makeup. The test can be done as early as 10 weeks of pregnancy.
Chorionic villus sampling can reveal whether a baby has a chromosomal condition, such as Trisomy 18 or Down syndrome, as well as other genetic conditions, such as cystic fibrosis.
Amnio (Amniocentesis, FISH test)
Amniocentesis is a procedure used to take out a small sample of the amniotic fluid for testing. This is the fluid that surrounds the fetus in a pregnant woman. Amniotic fluid is a clear, pale yellow fluid that:
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- Protects the fetus from injury
- Protects against infection
- Allows the baby to move and develop properly
- Helps control the temperature of the fetus
Along with various enzymes, proteins, hormones, and other substances, the amniotic fluid contains cells shed by the fetus. These cells have genetic information that can be used to diagnose genetic disorders, such as Trisomy 18, and open neural tube defects (ONTDs), such as spina bifida.
The fluid is sent to a lab so that the cells can grow and be analyzed. Results are most often ready in about 10 days to 14 days, depending on the lab.
Knowing which test was used is important in deciding what your next steps are. If a screening test has resulted in an elevated risk of Trisomy 18, a diagnostic test can be performed to determine whether your child has Trisomy 18. If a diagnostic test indicates a Trisomy 18 diagnosis, you will be faced with making decisions about your child’s care.