Children and adults with Down syndrome have distinct facial features. Though not all people with Down syndrome have the same features, some of the more common features include:

Potentially serious complications — The most serious complications of Down syndrome include heart defects, blood disorders that can include leukemia (cancer of the blood), and immune system problems.

Heart defects — Approximately half of all babies with Down syndrome are born with (often repairable) heart defects. Usually, these defects affect the walls separating the four chambers of the heart.

Blood disorders — Down syndrome can cause blood cell abnormalities, including a form of blood cancer called leukemia.

Height and weight — Babies with Down syndrome are usually smaller than other babies, and they have smaller heads. They may also grow more slowly and may never reach the same heights that typical children do. Although children with Down syndrome grow less than usual, they tend to gain more weight, possibly from a slower metabolism. As a result, parents need to guard against obesity by encouraging exercise and, if necessary, limiting the amount of food the child eats.

Vision — Most children with Down syndrome have some sort of vision problem, such as nearsightedness, farsightedness, or astigmatism (an abnormal curvature of the eye that causes blurred vision). They may also have weak eye muscles (cross-eyed) or have abnormal eye movements that impair vision. Glasses can often correct these problems.

Intellectual disability — Almost all babies born with Down syndrome are intellectually disabled, but the degree of impairment can vary a lot. Despite the disability, most children with Down syndrome can learn basic tasks; they just take a little longer than other babies to do so.

3) Diagnostic tests- Diagnostic tests can determine whether a fetus has Down syndrome with nearly 100 percent accuracy. However, because these tests require using a needle to obtain a sample from within the uterus, they carry a small increased risk of miscarriage and other complications.
Diagnostic test options include:
a)chorionic villus sampling (CVS)- a prenatal diagnosis method in which a small biopsy of the placenta is taken for specific genetic testing. CVS is used to detect any condition that involves specific chromosomal abnormalities, like Down syndrome. This test is usually performed during the first trimester between 10-14 weeks of gestation.
b)amniocentesis- a prenatal diagnosis method in which a needle is inserted into the amniotic sac that surrounds the fetus. Amniocentesis is most often used to detect Down syndrome and other chromosomal abnormalities. This test is usually done in the second trimester, after 15 weeks of gestation.
                                        

Facial features. Most babies with Down syndrome have some of these characteristics:

Other physical characteristics. Babies and children with Down syndrome may have some of these characteristics:

There are no known behavioral or environmental factors that cause Down syndrome.

Down syndrome is usually caused by an error in cell division called “nondisjunction.” Nondisjunction results in an embryo with three copies of chromosome 21 instead of the usual two. Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate. As the embryo develops, the extra chromosome replicates in every cell of the body. This type of Down syndrome, Trisomy 21, accounts for 95% of cases.



Mosaicism

Mosaicism (or mosaic Down syndrome) exists when there are a mixture of two types of cells, some containing the usual 46 chromosomes and some containing 47. Those cells with 47 chromosomes contain an extra chromosome 21. Mosaicism is the least common form of Down syndrome. It accounts for only about 1% of all cases of Down syndrome. Research indicates that individuals with mosaic Down syndrome may have fewer characteristics of Down syndrome than those with other types of Down syndrome. However, broad generalizations are not possible due to the wide range of abilities people with Down syndrome possess.