After finding out you’re pregnant, you may have many thoughts and questions. Some might ask how they can learn more about their baby’s development. Fortunately, there are prenatal screenings that provide relevant, actionable information for a variety of chromosomal conditions (such as the common trisomies).
What are trisomies?
People usually have 23 pairs of chromosomes, one of each from each parent. But occasionally, unexpected chromosome changes can occur. These change happen sporadically, and not because of a family history. Overall, chromosomal changes affect approximately 1 in 150 pregnancies and are responsible for 50% of early pregnancy losses.1
One type of unexpected chromosome change happens when a baby is born with three, instead of two, of a certain chromosome. This is called a trisomy. Babies with trisomies can have serious health issues, including birth defects and intellectual disabilities.
Screening for certain trisomies is available as early as eight weeks into your pregnancy so you can know if your baby has a higher or lower chance for these conditions. If so, in consultation with your healthcare provider, you’ll be able to seek diagnostic testing and understand whether there are interventions that can help your baby.
Common trisomies
While relatively rare, the three most common trisomies are trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome).
Trisomy 21 (Down syndrome)
Down syndrome occurs when there are three copies of chromosome 21. Down syndrome is the most common trisomy in the United States, occurring in about 1 in every 640 live births.2
The symptoms of Down syndrome vary greatly. Common features include mild to moderate intellectual disability, delays in development, unique facial features, and possibly other medical conditions, such as heart defects. The average life span for someone with Down syndrome is more than 60 years, although this depends on the severity of an individual’s health issues.
Trisomy 18 (Edwards syndrome)
Edwards syndrome occurs when there are three copies of chromosome 18. This occurs in about 1 in 8,600 live births.3 The symptoms include physical birth defects (like heart defects), poor growth, feeding difficulties, breathing issues, and intellectual disability. This condition is considered life-limiting because most affected pregnancies are at increased risk for miscarriage and stillbirth. Unfortunately, most infants do not survive beyond the first year of life.
Trisomy 13 (Patau syndrome)
Patau syndrome occurs when there are three copies of chromosome 13. It occurs in about 1 out of 10,000 live births.4 Trisomy 13 is also considered a life-limiting condition. The symptoms often also include physical birth defects (including heart, brain, and kidney abnormalities) as well as global developmental delays and intellectual disabilities. As with trisomy 18, these pregnancies are at increased risk for miscarriage, stillbirth, or passing away during infancy.
Why to screen for trisomies
For all of these conditions, it is helpful to know before your delivery whether your baby is at risk. This information helps you and your healthcare provider understand your options. You might consider additional diagnostic tests, and if appropriate, identify specialists and facilities with the experience and equipment to provide the best care for your baby during pregnancy, delivery, and after birth.
How to screen for trisomies
Most babies will not have a trisomy. For those who do, it can be a serious condition. Approximately 50% of pregnancies with Down syndrome, and ~90% of pregnancies with trisomy 18 or trisomy 13 may have an abnormal finding identified on ultrasound. Ultrasound , however, cannot diagnose any trisomy.5 Therefore, prenatal cell-free DNA (cfDNA) screening, in addition to ultrasound, is the most sensitive way to help you screen your pregnancy and know if your baby is at risk.
A prenatal cfDNA screen is a blood test that analyzes small fragments of the baby’s DNA in the pregnant person’s blood to determine the chance of certain chromosome conditions. CfDNA screening can take place as early as eight weeks into pregnancy.6
If a screening result turns up higher risk, your healthcare provider may suggest a follow-up diagnostic test.6 These tests include:
- Chorionic villus sampling (CVS). Starting in the 15th week of pregnancy, a doctor can take a small sample of your placenta to confirm the result of the cfDNA screen.
- Amniocentesis. Between the 16th and 22nd week of pregnancy, a doctor can take a small sample of fluid from your uterus to confirm the result of the cfDNA screen.
The American College of Obstetricians and Gynecologists recommends cfDNA screening for all pregnant people, regardless of age or risk factors.7 Prenatal cfDNA screenings are non-invasive and offer a high degree of accuracy, and most of the time, peace of mind.8
- LeFevre NM, Sundermeyer RL. Fetal aneuploidy: screening and diagnostic testing. Am Fam Physician. 2020;101(8):481-488.
- CDC. Living with Down syndrome. http://www.cdc.gov/birth-defects/living-with-down-syndrome/. November 22, 2024. Accessed March 31, 2025.
- Balasundaram P, Avulakunta ID. Edwards syndrome [updated online March 20, 2023]. StatPearls. 2025 Jan-. https://www.ncbi.nlm.nih.gov/books/NBK570597/. Accessed March 31, 2025.
- Myriad Genetics. Trisomy 13. http://myriad.com/womens-health/prequel-diseases/trisomy-13/. Accessed March 31, 2025.
- Witters G, Van Robays J, Willekes C, Coumans A, Peeters H, Gyselaers W, Fryns JP. Trisomy 13, 18, 21, Triploidy and Turner syndrome: the 5T’s. Look at the hands. Facts Views Vis Obgyn. 2011;3(1):15-21. PMID: 24753843; PMCID: PMC3991414.
- The American College of Obstetricians and Gynecologists. Cell-free DNA prenatal screening test. https://www.acog.org/womens-health/infographics/cell-free-dna-prenatal-screening-test. Accessed March 31, 2025.
- American College of Obstetricians and Gynecologists. Cell-free DNA Screening for Fetal Aneuploidy. ACOG. https://www.acog.org/clinical/clinical-guidance/practice-bulletin/articles/2020/10/screening-for-fetal-chromosomal-abnormalities. Published October 2020. Accessed April 22, 2025.
- Riggan KA, Barwise A, Yap JQ, Condon N, Allyse MA. Patient experiences with prenatal cell-free DNA screening in a safety net setting. Prenat Diagn. 2024;44(4):409-417. doi:10.1002/pd.6541
REFERENCES

Learn about the screening process and how to talk to your doctor.
How to get screened