Trisomy 21 (Down Syndrome)


Trisomy 21, commonly known as Down syndrome, is a genetic disorder that causes intellectual disability, developmental delays, certain facial features, and other birth defects and health issues.

The condition occurs when the baby has an extra chromosome. This extra genetic material alters how the baby’s brain and body develop. The degree of impairment caused by Down syndrome varies with each child and can range from mild to severe.

Down syndrome is one of the most common genetic conditions, affecting an estimated 1 in every 700 babies born in the United States, according to the Centers for Disease Control and Prevention (CDC). Trisomy 21 due to an extra copy of chromosome 21 is the most common type of Down syndrome, accounting for 95% of all cases.

What causes Trisomy 21?

Typically, a baby is born with 46 chromosomes in every cell in their body. These chromosomes are arranged in 23 matching pairs – one in each pair comes from the mother, the other from the father. Chromosomes contain our DNA, or genes, which are the instructions for the human body – how we look and how our body develops and functions.

Babies with Trisomy 21 have an extra copy of chromosome 21. Instead of the usual pair, there are three copies, for a total of 47 chromosomes in every cell. This additional genetic material changes the course of cell development, causing the characteristics associated with Down syndrome.

Three Types of Down Syndrome

Trisomy 21 is the most common type of Down syndrome. It is sometimes referred to as “nondisjunction,” a term used to describe chromosomes that are unable to separate (or disjoin) properly.

How will Trisomy 21 affect my baby?

While each child’s condition is unique, common characteristics and health issues associated with Down syndrome include:

  • Distinctive facial features such as almond-shaped eyes that slant upward, a small nose with a flattened bridge, and a small mouth with a tongue that may stick out
  • Intellectual disability that is typically mild to moderate
  • Learning difficulties
  • Birth defects, most commonly congenital heart defects
  • Short height
  • Weak muscle tone (known as hypotonia) and loose joints
  • A short neck
  • Small ears
  • Small hands and feet, and a single line across the palm of the hand (palmar crease)
  • Gastrointestinal tract problems
  • Thyroid conditions
  • Speech that develops slower than other children
  • Vision and hearing problems
  • Sleep disorders
  • Airway problems
  • Increased risk of infection and of leukemia


During pregnancy

There are two types of prenatal testing for Down syndrome. Both are optional.

Screening tests are used to check the risk of your baby having Down syndrome. These tests are noninvasive, using a combination of ultrasounds and a maternal blood sample.

Diagnostic tests are used to detect if your baby has Down syndrome. Diagnostic tests may be performed after a positive screening test, meaning your baby is at high risk, or they may be recommended if the mother is 35 or older, because of the increased risk of having a baby with Down syndrome.

Diagnostic tests involve taking a sample of genetic material and analyzing the chromosomes. Routine diagnostic tests include:

  • Chorionic villus sampling (CVS) – genetic testing of a sample of tissue taken from the placenta in the first trimester
  • Amniocentesis – genetic testing of a sample of the amniotic fluid taken from the womb in the second trimester

Diagnostic testing carries a small risk of complications, including the risk of pregnancy loss. Talk with your doctor about the risks involved before undergoing testing.

At Birth

If Down syndrome isn’t diagnosed during pregnancy, the condition may be suspected at birth following a physical exam of a newborn with features associated with the disorder. Genetic testing using a blood sample from the baby can confirm the diagnosis.

Specialized Prenatal Evaluation and Care

If Trisomy 21 is suspected or diagnosed, you may be referred to a fetal center for specialized care and evaluation.

At Texas Children’s Fetal Center, we arrange for you to visit as quickly as possible for a comprehensive assessment by a team of specialists experienced in the diagnosis and treatment of Down syndrome and associated birth defects or health issues. This team includes maternal-fetal medicine physicians (OB/GYNs specializing in high-risk pregnancies), fetal imaging experts, geneticists, genetic counselors, neonatologists, and pediatric cardiologists and neurologists.  

Additional testing may be performed using state-of-the-art imaging technologies to gain critical information about your baby’s condition, including any associated abnormalities.

Our specialists will then meet with you about the results, provide treatment recommendations, and answer any questions your family has to help you make the most informed decisions regarding your baby’s care.

A diagnosis during pregnancy enables your family and your healthcare team to plan ahead for the specialized care your baby may need at birth.

Mother and baby will be closely monitored throughout pregnancy with regular ultrasounds to assess fetal growth and watch for signs of complications.


Delivery at a center with the resources and expertise to care for newborns with Down syndrome and any associated defects is recommended, including the highest level neonatal intensive care unit (NICU), if needed. Babies with Down syndrome are often born prematurely with a lower than normal birth weight.

Delivery and postnatal care should be carefully planned and coordinated with a team of maternal-fetal medicine physicians, neonatologists, and pediatric specialists experienced in working together to treat babies with Down syndrome. Our Fetal Center team works closely with pediatric experts from the Down Syndrome Clinic at Texas Children’s Hospital, consistently ranked one of the best children’s hospitals in the nation by U.S. News & World Report.

Treatment After Birth

Treatment after birth will depend on each child’s unique needs.

Some newborns with Down syndrome may require a stay in the NICU, such as those with heart defects or other congenital anomalies, respiratory issues, feeding difficulties, or low birth weight due to prematurity.

In general, treatment strategies for children with Down syndrome may include:

  • A physical exam, blood tests, imaging, and genetic testing at birth if not performed prenatally
  • Medical or surgical treatment of any birth defects or health issues present
  • Early intervention to help the child reach their full physical and intellectual potential, with specialized therapies tailored to the child’s needs
  • Ongoing monitoring for early detection and treatment of new health complications or developmental issues that may emerge as the child ages
  • Recommended screening tests for medical conditions common to children with Down syndrome

Postnatal Care Team

Children with Down syndrome typically require a team of specialists and healthcare professionals to address their physical and intellectual needs.

While the needs vary for each child, in general a postnatal care team may include:

Why Texas Children’s Fetal Center?

  • A single location for expert maternal, fetal, and pediatric care. At Texas Children’s Hospital, mother and baby receive the specialized care required for the diagnosis and treatment of Down syndrome  all in one location, including immediate access to our level IV NICU, if needed.
  • A skilled, experienced team with proven outcomes. We have a dedicated team of maternal-fetal medicine specialists, fetal imaging experts, geneticists and genetic counselors, neonatologists, pediatric cardiologists and neurologists, and others who work in concert to care for you and your baby every step of the way, using proven protocols we’ve developed over the years. With their combined expertise and unified approach, this team offers the best possible care for babies with Trisomy 21.
  • We care for your child’s needs at every stage of life. Our comprehensive approach starts with your first prenatal visit and continues through delivery, postnatal care, childhood and adolescence, thanks to one of the nation’s leading teams of fetal and pediatric specialists for the care and treatment of chromosomal disorders.


Patient Stories

Additional Resources

For more information or to schedule an appointment,

call Texas Children’s Fetal Center at 832-822-2229 or 1-877-FetalRx (338-2579) toll-free.

Our phones are answered 24/7. Immediate appointments are often available.