- April 21, 2020
- Michael Allibone
- Posted in Resources
From the moment you find out you're pregnant, you want to know more about the baby growing inside you. While you're full of hopes and dreams for your developing little one, you also want to be sure he or she is growing normally. That's where prenatal genetic testing comes in.
You've undoubtedly heard of Down syndrome, but you may not be as familiar with its other name: trisomy 21. Trisomy 21, along with trisomy 18 and trisomy 13, are genetic conditions that are commonly tested for during pregnancy.
So, what is trisomy? What is the difference between Down syndrome, trisomy 18, and trisomy 13? And are there prenatal screening tests that can tell you whether your baby is at risk for these chromosomal abnormalities?
We'll answer each of these questions so you have all the information you need to decide whether genetic testing is right for you.
What Is Trisomy?
Do you remember that week in biology class where you learned about genetics? If so, now's your chance to put that knowledge to use!
You may remember learning about chromosomes, which are the threads of genetic material that make us who we are. Chromosomes are responsible for everything from our eye and hair color to which diseases we'll be susceptible to as we age.
Normally, we humans have 23 pairs of chromosomes in their DNA. Trisomy is a genetic disorder in which a person has three copies of a particular chromosome instead of the usual set of two.
Since scientists have numbered our chromosomes 1 through 23, the name of the condition – trisomy 21, trisomy 18, or trisomy 13 – indicates the specific chromosome that carries the abnormality. For example, in the case of Down syndrome (trisomy 21), there are three copies of chromosome number 21.
What Is Down Syndrome?
Genetically, people with Down syndrome have an extra copy of chromosome 21. In most cases, Down syndrome is caused by a random abnormal creation of the genes found in the egg or the sperm. It is not an inherited condition.
People with this condition typically have an intellectual disability, characteristic facial features (such as wide-set eyes), and weak muscle tone in infancy. Babies with Down syndrome may also have birth defects that affect their hearts and digestive systems.
Although most of the time Down syndrome isn't an inherited condition, there are a few known risk factors. Mothers over 35 are at particular risk of having babies with Down syndrome because a mother's aging eggs may divide abnormally and cause an extra copy of chromosome 21.
Another risk factor is if the mother or the father is a carrier for the condition. Such a scenario is uncommon, but it can lead to Down syndrome.
Lastly, if you already have one child with Down syndrome, your risk of having additional children with the condition is higher. A genetic counselor can help you determine your individual risk factors.
What Is Trisomy 18?
Trisomy 18 is caused by an extra copy of chromosome 18. It's also known as Edwards syndrome, named after the physician who first diagnosed the condition. Like Down syndrome, trisomy 18 usually occurs because of a random genetic event rather than an inherited condition.
Trisomy 18 is more life-threatening than Down syndrome. Before they're born, babies with trisomy 18 experience a slow growth rate as well as heart defects and other organ abnormalities. According to the National Institutes of Health, once born, they also tend to have a low birth weight, "small, abnormally-shaped head, small jaw and mouth, and clenched fists with overlapping fingers."
Severe medical problems caused by heart defects and organ abnormalities threaten their lives from the very beginning. Sadly, most babies with trisomy 18 die before birth or within their first month of life. Only "five to 10 percent of children with this condition live past their first year, and these children often have severe intellectual disability," according to NIH sources.
The risk factors for trisomy 18 are the same as those for Down syndrome.
What Is Trisomy 13?
An extra copy of chromosome 13 causes trisomy 13, also known as Patau syndrome. As with Down syndrome and trisomy 18, trisomy 13 usually occurs due to a random genetic event.
Babies with trisomy 13 are born with life-threatening medical conditions, including severe intellectual disabilities and terrible physical abnormalities. According to Genetic and Rare Diseases Information Center (GARD), they usually have heart defects, brain or spinal cord irregularities, very small or poorly developed eyes, extra fingers or toes, a cleft lip, cleft palate, and weak muscle tone.
Sadly, babies with trisomy 13 often die within their first few days or weeks of life. Only five to ten percent of babies with trisomy 13 will live past their first birthday, says GARD.
Trisomy 13 has the same risk factors as Down syndrome and trisomy 18, and advanced maternal age is the biggest predictor of the condition.
Are There Prenatal Screening Tests for Trisomies?
With so many frightening abnormalities associated with trisomies, it's only natural to want to know whether there is a risk that your baby is born with one of these conditions.
NTD Eurofins offers several non-invasive prenatal screening tests. Each can tell you whether you're at a greater risk for the three types of trisomy.
Verifi Prenatal Screen
With the prenatal screening test Verifi, your OB will draw a sample of your blood as early as 10 weeks into your pregnancy. The test will then analyze the DNA present in your blood and is capable of determining how many chromosomes are present.
Conveniently, Verifi can also reveal the sex of your baby!
First Trimester Prenatal Screen
Another testing option offered by NTD Eurofins is the First Trimester Prenatal Screen. This test requires a sample of your blood, in addition to an ultrasound performed by your OB sometime between nine and 13 weeks into your pregnancy.
During this ultrasound, your OB will also check for fluid accumulation behind your baby's neck, which can indicate Down syndrome.
The First Trimester Prenatal Screen analyzes the amount of alpha-fetoprotein, free beta hCG, and pregnancy-associated plasma protein-A in your blood. NTD Eurofins is the only lab that utilizes free beta hCG, a very accurate predictor of Down syndrome, in their screening test for trisomy conditions.
Maternal Fetal T1 Screen
If you're looking for a prenatal screening test that will cover a wide variety of medical conditions, then NTD Eurofins' Maternal Fetal Screen is the best choice. It's an enhanced prenatal screening test offered between 10 and 13 weeks into your pregnancy.
The Maternal Fetal T1 Screen combines ultrasound measurements with five biological markers in your blood sample. It can determine your risk probabilities for conditions including:
- Down syndrome
- Trisomy 18
- Trisomy 13
- Low birth weight
- Preterm birth
- Ventral wall defects
- Placental accreta
- Congenital adrenal hypoplasia
- Sex chromosome aneuploidy
- Open spina bifida
- Intrauterine growth restriction
- Fetal loss
- Steroid sulfatase deficiency
This screening test can also determine the sex of your baby much earlier than the usual 20-week ultrasound.
If you're concerned about genetic conditions, then a prenatal screening test can go a long way toward easing your mind. Learn more about trisomy screening tests from NTD Eurofins.
- U.S. National Library of Medicine. (2020). Trisomy 18. National Institutes of Health. Genetics Home Reference. https://ghr.nlm.nih.gov/condition/trisomy-18
- GARD. (2015). Trisomy 13. National Institutes of Health. Genetic and Rare Diseases Information Center (GARD). https://rarediseases.info.nih.gov/diseases/7341/trisomy-13
The information provided represents the general opinions of NTD Eurofins and is not intended to be used as specific advice for any one individual. Individuals should always consult with a physician to obtain specific advice and to receive answers to any and all questions or concerns related to health, wellness, pregnancy, and birth.
Pursuant to applicable federal and/or state laboratory requirements, Eurofins NTD, LLC has established and verified the accuracy and precision of its testing services. Tests are developed and performance characteristics determined by Eurofins NTD, LLC. The methods and performance characteristics have been reviewed and approved by the New York State Department of Health.