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  • Elizabeth Kearney, MS, CGC

What people planning for pregnancy should know about Down syndrome screening

Prenatal screening for Down syndrome has been offered for many decades. What is Down syndrome and why is screening offered to every pregnant woman?



What is Down syndrome?


Down syndrome is a genetic condition that includes some physical conditions, like certain facial features, a higher chance of certain medical problems, and some degree of intellectual disability. (Formerly, this type of disability was called mental retardation, but that term has been replaced because of its stigma.) Intellectual disability means that a person learns more slowly and in a different way than the average person. However, learning continues throughout life, and people with Down syndrome can do many of the things that other people do, including hold jobs, go to school, and form rich relationships with family and friends.


In addition to intellectual disability, people with Down syndrome have a higher chance for certain medical conditions. For example, heart defects are more common and vary in whether they require surgical intervention. People with Down syndrome also have a higher chance for medical conditions like thyroid problems, hearing loss, and some instability in their spinal cord. Most of these conditions can be managed with good healthcare. Screening is recommended for these health issues to catch them early and provide medical care, allowing for people with Down syndrome to reach their greatest potential.


What causes Down syndrome?


Down syndrome is caused by an extra copy of a chromosome. Chromosomes are inside cells and contain all the genetic information people need for their bodies to develop, grow and function. Most people have 46 chromosomes, or 23 pairs, one from the mother and one from the father. People with Down syndrome have an extra copy of chromosome number 21. The medical term for an extra chromosome is “trisomy”, so Down syndrome is also called trisomy 21.

Why does a pregnant woman’s age matter in terms of the chances to have a child with Down syndrome?


Any woman can have a child with Down syndrome, regardless of her age. However, as women get older, there is a higher chance for them to have a baby with an extra or missing chromosome. Nothing magical happens at age 35; the chance for a chromosomal problem is slowly increasing with a woman’s age throughout her life.


Women are born with all of their eggs. As the eggs get older, the eggs are more likely to have an extra or missing copy of a chromosome. If the extra chromosome is a No. 21 chromosome, the child will have Down syndrome if the egg is fertilized.


Is Down syndrome inherited?


Down syndrome is not usually inherited in the sense that is passed from one generation to the next. In fact, even a couple who has already had a child with Down syndrome have only a 1% chance until the woman reaches 38 years of age. At age 38, the couple's chance for Down syndrome in the next pregnancy is considered equal to her age-related chances for Down syndrome.


However, a small percentage of cases result from a chromosome translocation. In a chromosomal translocation causing Down syndrome, the extra 21 chromosome is attached to another chromosome. In those cases, it is possible one of the parents has a chromosomal translocation but still only has two copies of chromosome 21. Obtaining a copy of the test results that originally diagnosed the family member with Down syndrome can show whether the person has a translocation form of Down syndrome or the more typical trisomy 21 with three distinct copies of chromosome 21. If there is a translocation in the family, family members can also have blood testing to find out if they carry a chromosomal translocation that could mean a higher chance to have children with Down syndrome.


Why should people planning for pregnancy know about Down syndrome?


About 1 in 700 babies has Down syndrome, making it one of the more common types of developmental differences and the most common chromosomal condition. But the reason people planning for pregnancy should know about Down syndrome is because screening for Down syndrome and some other chromosomal conditions is offered to most pregnant women. The screening is not required though, so it is important for people who are pregnant or considering pregnancy to understand how Down syndrome screening works and whether it is right for them.


What is prenatal screening for Down syndrome and what should people planning for pregnancy know about it?


A special genetic screen called non-invasive prenatal testing, or NIPT, is offered to most pregnant women in the first or second trimester of pregnancy. (Sometimes NIPT is also called NIPS. The “s” stands for “screening”.) A sample of a woman’s blood is taken, and a laboratory analyzes her blood. The lab then determines the likelihood of Down syndrome and some other chromosome problems and provides that to the patient and her doctor in a test report.


Pregnant women’s blood has tiny amounts of genetic material from their babies. The placenta, or the membrane surrounding the baby during pregnancy, contains the same genetic material, or DNA, as the baby, and some fragments of DNA get loose in moms’ blood from the outer cells of the placenta. Those fragments of DNA can be studied in a laboratory in order to predict the likelihood that the fetus has the typical number of chromosomes.


A screen positive result with NIPT means that the chances that the baby has a chromosome problem is higher. The woman would be offered an amniocentesis or CVS to know for sure.

Similarly, a screen negative results with NIPT is reassuring but cannot rule out Down syndrome or other extra or missing chromosomes completely. There are also other forms of birth defects, developmental problems, and inherited diseases that are not included with NIPS. It’s important to remember that most pregnancies result in a healthy baby; all of these genetic screening tests are optional and meant to be helpful for people who want to know more about their babies’ health.


There are other screening tests for Down syndrome that are sometimes offered during pregnancy. For example, a measurement of the nuchal fold, which is the area at the back of the baby’s neck, can be taken to assess the chances for Down syndrome late in the first trimester. Also, a different blood test can be used late in the first trimester or during the second trimester. This type of test is typically called “serum screening” and does not look at the baby’s DNA. Instead it measures other blood factors that suggest the chances for Down syndrome, too. These tests were used for many years before NIPT became available and some prenatal care providers still use them as reliable screening tests.


Is there a way to know for sure during pregnancy whether the baby has Down syndrome?


Amniocentesis or chorionic villus sampling (CVS) are the tests used to diagnose Down syndrome in a baby during pregnancy. (These are called diagnostic tests compared to the screening tests described above.)


Chorionic villus sampling (CVS) is done in the late first trimester. Amniocentesis is done in the second trimester. During CVS, a catheter is inserted through a woman’s cervix to obtain a small amount of tissue from the placenta. With amniocentesis, a thin needle is inserted through the mother’s abdominal wall and uterine wall to obtain a small amount of the fluid around the baby. (With CVS and amniocentesis, the needle does not touch the developing baby because the tissue or fluid contain cells, which have DNA.) A laboratory uses the tissue or fluid to look at the baby’s chromosomes.


How do people who are pregnant or planning for pregnancy know whether Down syndrome screening is right for them?

A pregnant woman and her partner have the choice whether they want to have Down syndrome screening and screening for other chromosomal or genetic conditions. Not everyone has to have screening.


Some reasons why people have Down syndrome screening or screening for other genetic conditions is if they would consider options in a pregnancy or if they want to plan ahead for a child with special needs.


Prospective parents of a child with Down syndrome may want to learn about the condition before the baby arrives and demands the attention that any new baby requires. This learning might include reviewing materials from organizations that provide advocacy for people with Down syndrome or connecting with other parents of children with Down syndrome. It may mean finding a pediatrician who has experience with caring for children with Down syndrome or who is open to learning about the special medical needs of a child with Down syndrome. There are also special needs programs run through states that might be helpful to research in advance.


Other couples may decide based on their particular situation and resources that they cannot care for a child with Down syndrome. Special needs adoption programs are available. Others may decide to end a pregnancy, which is legal in many states.


If a woman has had carrier screening, does she still need to consider screening for Down syndrome during her pregnancy?


Carrier screening does not screen for Down syndrome. Carrier screening is for a different group of genetic conditions that are inherited from parents who are healthy, but carriers of the conditions. Our blog post about carrier screening contains more details about the benefits and limitations of carrier screening.


It can be confusing because often carrier screening is often offered at the same time as NIPT is offered during pregnancy. However, carrier screening can be done at other times and provides the most number of family planning options if it is done preconception, or prior to pregnancy.


If a couple planning for pregnancy has no one in their family with Down syndrome or with birth defects, can they still have a child with Down syndrome?


The fact that a couple has no family members with Down syndrome does not mean they have no chance to have a child with Down syndrome themselves. The best predictor of the chances is the mother’s age. (See the table above.)


However, other birth defects or problems with development run in families, and family medical history can be an important factor to consider. The Pregnancy Planning Family Screen is a good place to start to learn whether there is any other screening or testing a couple planning for pregnancy might want to consider.



If a woman has had Down syndrome screening and did not have a high risk, does that mean her baby is healthy?


If a woman does not have a high risk for Down syndrome on her prenatal screening test, she has a lower chance for Down syndrome specifically. It does not completely rule out the chance for Down syndrome or other chromosomal problems in her baby. Also, about 3-5% of babies have birth defects or problems with development, and Down syndrome is only one type of developmental difference.


The greatest odds for most pregnancies is a healthy baby!

The reason so much information is given about Down syndrome is because screening is offered. Prenatal care providers want their patients to have the right information and make an informed choice. It is a woman’s or couple’s choice whether they want to have any type of genetic screening during or before a pregnancy. The more people planning for pregnancy understand their options and consider their choices, the better they can explain their choices to their prenatal care providers.


Sources:

1. What is Down Syndrome? National Down Syndrome Society. Accessed March 10, 2021. https://www.ndss.org/about-down-syndrome/down-syndrome/


2. Facts about Down syndrome. Center for Disease Control. Accessed March 11, 2021.

https://www.cdc.gov/ncbddd/birthdefects/downsyndrome.html


3. Screening for Fetal Chromosomal Abnormalities. ACOG Practice Bulletin. October 2020.

https://www.acog.org/clinical/clinical-guidance/practice-bulletin/articles/2020/10/screening-for-fetal-chromosomal-abnormalities



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