How Many Chromosomes Are in Down Syndrome? What Ultrasound Signs Can Be Seen During Pregnancy?

Down syndrome, also known as trisomy 21, is a congenital condition that can lead to intellectual disabilities and other developmental and health challenges. It’s caused by a genetic abnormality that is present from birth, and while there is currently no cure, effective management and support can greatly improve quality of life.

Most individuals with Down syndrome share similarities in physical features, developmental patterns, susceptibility to certain illnesses, cognitive abilities, and life expectancy. Compared with other congenital conditions, Down syndrome is one of the earliest chromosomal disorders to be identified and also one of the most thoroughly studied.

In Taiwan, approximately 92% of Down syndrome cases are caused by an extra copy of chromosome 21, 4% by chromosomal translocation, and another 4% are classified as mosaic Down syndrome.

How many chromosomes are there in Down syndrome?

Down syndrome is the most common chromosomal disorder, caused by an abnormality in the 21st pair of chromosomes. The incidence rate is about 1 in 800, meaning that approximately one in every 800 newborns has Down syndrome.

Why does a baby develop Down syndrome?

However, if an error occurs during the formation of reproductive cells, a sperm or egg may end up with an extra chromosome. When this happens to the 21st pair, the fertilized egg has three copies instead of the usual two. This extra genetic material disrupts normal development, leading to the characteristics of Down syndrome.
Medical studies have shown that this abnormality is primarily related to the aging of the egg. As a woman gets older, her eggs are more likely to have errors during meiotic division. For this reason, maternal age is considered a significant risk factor, and statistics show that older mothers have a higher chance of having a baby with Down syndrome.

Three Types of Chromosomal Variations in Down Syndrome

1. Trisomy 21 (Nondisjunction): The most common type, accounting for about 95% of cases.
2. Translocation: A small portion of an extra chromosome 21 becomes attached to another chromosome.
3. Mosaicism: A mixture of normal and abnormal cells within the body.

Can Down syndrome be detected through pregnancy ultrasounds?

In early pregnancy, one of the first signs doctors look for is an increased nuchal translucency (NT)—a fluid-filled space at the back of the baby’s neck. In a healthy fetus, this space typically measures less than 2.5 to 3 millimeters on an ultrasound. A thicker NT may indicate a higher risk of chromosomal abnormalities like Down syndrome, and doctors use it as a key indicator for early screening.

During ultrasound examinations, doctors may also observe other features, such as a shorter nasal bone, or in some cases, the nasal bone may be absent.

However, the length of the nasal bone alone cannot confirm Down syndrome, as nasal bone size can also be influenced by parental genetics. It is only one possible indicator and should be assessed together with other screening results.

What are the features of a baby with Down syndrome?

Children with Down syndrome generally develop more slowly, both mentally and physically, compared to other children of the same age. While some newborns with Down syndrome may have an average weight and length at birth, their growth and developmental milestones often lag behind as they grow older.

They tend to share a range of distinctive physical features, such as a shorter neck, slightly unusual ear shape, a tongue that protrudes, a smaller head size, a flatter facial profile, white spots on the iris, shorter fingers, and a single deep crease across the palm.
Because their limbs are usually shorter, many also have lower muscle tone. Some may experience looseness in the tongue and ligaments, leading to a drooping appearance.

Babies with Down syndrome are also more prone to certain health conditions, including hearing problems, congenital heart disease, intestinal blockages, abdominal issues, cataracts, thyroid disorders, skeletal abnormalities, Alzheimer’s disease, and immune system problems that make them more vulnerable to infections like pneumonia.
In addition, they may be at higher risk of developmental and emotional challenges. Some children with Down syndrome may show traits such as stubbornness, short attention span, compulsive behaviors, or a tendency toward anxiety.

Is the risk of Down syndrome high? Will my next baby also have Down syndrome?

Every expectant mother has a chance of giving birth to a baby with Down syndrome, and the risk increases with maternal age.

Statistics show that at age 20, the chance is about 1 in 1,528; at age 30, it rises to 1 in 909; and for mothers aged 34 and above, the risk further increases to around 1 in 270.

Because of this, Taiwan’s Ministry of Health and Welfare recommends that women aged 34 or older consider amniocentesis as part of prenatal screening. This test analyzes chromosomes from amniotic fluid and provides more accurate, diagnostic results.

For women under 34, the individual risk is lower, but since this age group makes up the majority of pregnant women, about 80% of babies with Down syndrome are actually born to mothers under 34. For this reason, medical experts also strongly advise prenatal screening for younger mothers, so that any potential risks can be identified early and appropriate follow-up decisions can be made.

In short, regardless of age, every pregnant woman has some risk. That’s why prenatal screening and proper diagnostic testing are so important for protecting both mother and baby’s health.

Is there a way to prevent having a baby with Down syndrome?

Currently, the main medical approach to lowering the risk is through pre-pregnancy health checks and prenatal diagnostic tests.
Statistics show that in Taiwan, about 1 in every 800 newborns is born with Down syndrome. Since chromosomal abnormalities in Down syndrome cannot be prevented with vaccines, and there is no medicine or treatment that can cure it, prenatal screening remains the only effective way to detect and reduce the risk early on.

Common Prenatal Screening Methods

• First- and second-trimester Down syndrome screenings
• Non-invasive prenatal testing (NIPT)
• Amniocentesis: A small amount of amniotic fluid (about 10 c.c.) is collected to examine the baby’s chromosomes. This can be combined with microarray analysis to detect small chromosomal deletions or other abnormalities.

• Reference：https://www.cmuh.cmu.edu.tw/HealthEdus/Detail?no=5256