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2025.08.31

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.

Frequently Asked Questions About 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

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

<< Contact us >>
For overseas patients, please leave a message below or Email to ivftaiwan@gmail.com
For patients in Hong Kong and Macau, please Email to ivftaiwan.hk@gmail.com
For patients in China, please add our official WeChat account: ivftaiwan01
★Any immediate or urgent inquiry, please dial +886-4-22347057 #1324

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.

Will my next baby also have Down syndrome?

As mentioned, there are three types of chromosomal variations in Down syndrome: trisomy 21, translocation, and mosaicism. Medical studies show that if the first baby with Down syndrome is diagnosed with the translocation type, the chances of having another baby with Down syndrome in a future pregnancy are higher, since one of the parents may carry the translocated chromosome.

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.

These tests allow doctors to evaluate risks early in pregnancy and determine whether the baby has Down syndrome or other chromosomal conditions.

Although there is currently no medication to cure Down syndrome, advances in research and medical care have made early intervention and therapy an important part of treatment.

For example, babies with Down syndrome often have low muscle tone. Through early intervention programs such as strength and movement training, they can improve muscle endurance and gradually develop self-care skills. With long-term support, these therapies help increase their ability to live more independently as they grow.

Reference：https://www.cmuh.cmu.edu.tw/HealthEdus/Detail?no=5256
Reviewing Physician: Dr. Wu Ting-feng

<< Contact us >>
For overseas patients, please leave a message below or Email to ivftaiwan@gmail.com
For patients in Hong Kong and Macau, please Email to ivftaiwan.hk@gmail.com
For patients in China, please add our official WeChat account: ivftaiwan01
★Any immediate or urgent inquiry, please dial +886-4-22347057 #1324

<p>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.<br />
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.</p>

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

<div class="fast-list">
<h3 class="h3title">Frequently Asked Questions About Down Syndrome</h3>

<h2 class="h2title">How many chromosomes are there in Down syndrome?</h2>

<p>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.</p>

<h2 class="h2title">Why does a baby develop Down syndrome?</h2>

<p>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.<br />
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 <a href="https://ivf.ivftaiwan.com/share-detail/advanced-maternal-age/">older mothers</a> have a higher chance of having a baby with Down syndrome.</p>

<h3 class="h3title">Three Types of Chromosomal Variations in Down Syndrome</h3>

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

<blockquote style="border-left: 4px solid #339900; padding-left: 15px; margin: 20px 0; font-style: italic;"><< <a href="http://ivf.ivftaiwan.com/contact/" target="_blank">Contact us</a> >><br />
For overseas patients, please leave a message below or Email to <a href="mailto:ivftaiwan@gmail.com">ivftaiwan@gmail.com</a><br />
For patients in <strong>Hong Kong</strong> and <strong>Macau</strong>, please Email to <a href="mailto:ivftaiwan.hk@gmail.com">ivftaiwan.hk@gmail.com</a><br />
For patients in <strong>China</strong>, please add our official WeChat account: <a href="https://lihi-io.s3.us-west-004.backblazeb2.com/dXNlcl82NzY5NQ==/1704870538341.jpg" target="_blank">ivftaiwan01</a><br />
★Any immediate or urgent inquiry, please dial +886-4-22347057 #1324</blockquote>

<h2 class="h2title">Can Down syndrome be detected through pregnancy ultrasounds?</h2>

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

<h2 class="h2title">What are the features of a baby with Down syndrome?</h2>

<p>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.<br />
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.<br />
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.<br />
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.<br />
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.</p>

<h2 class="h2title">Is the risk of Down syndrome high? Will my next baby also have Down syndrome?</h2>

<p>Every expectant mother has a chance of giving birth to a baby with Down syndrome, <strong>and the risk increases with maternal age.</strong><br />
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.<br />
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.<br />
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.<br />
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.</p>

<div class="keypoint">
<h3 class="h3title">Will my next baby also have Down syndrome?</h3>

<p>As mentioned, there are three types of chromosomal variations in Down syndrome: trisomy 21, translocation, and mosaicism. Medical studies show that if the first baby with Down syndrome is diagnosed with the translocation type, the chances of having another baby with Down syndrome in a future pregnancy are higher, since one of the parents may carry the translocated chromosome.</p>
</div>

<h2 class="h2title">Is there a way to prevent having a baby with Down syndrome?</h2>

<p>Currently, the main medical approach to lowering the risk is through pre-pregnancy health checks and prenatal diagnostic tests.<br />
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.</p>

<h3 class="h3title">Common Prenatal Screening Methods</h3>

<ul>
	<li>
	<div>First- and second-trimester Down syndrome screenings</div>
	</li>
	<li>
	<div>Non-invasive prenatal testing (NIPT)</div>
	</li>
	<li>
	<div>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.</div>
	</li>
</ul>

<div>These tests allow doctors to evaluate risks early in pregnancy and determine whether the baby has Down syndrome or other chromosomal conditions.</div>

<div>Although there is currently no medication to cure Down syndrome, advances in research and medical care have made early intervention and therapy an important part of treatment.</div>

<div>For example, babies with Down syndrome often have low muscle tone. Through early intervention programs such as strength and movement training, they can improve muscle endurance and gradually develop self-care skills. With long-term support, these therapies help increase their ability to live more independently as they grow.</div>

<ul>
	<li>Reference：<a href="https://www.cmuh.cmu.edu.tw/HealthEdus/Detail?no=5256">https://www.cmuh.cmu.edu.tw/HealthEdus/Detail?no=5256</a></li>
	<li>Reviewing Physician: <a href="https://ivf.ivftaiwan.com/member-detail/Ting-Feng-Wu/" target="_blank" title="吳亭鋒醫師">Dr. Wu Ting-feng</a></li>
</ul>

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