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Prenatal Diagnosis of Down Syndrome

Health care providers can check for Down syndrome during pregnancy or after a child is born. 
The American Congress of Obstetricians and Gynecologists (ACOG) recommends that all pregnant women be offered a Down syndrome screening test.
There are two types of tests for Down syndrome during pregnancy:

Prenatal Screening for Down Syndrome

There are several options for Down syndrome prenatal screening. These include:
If a woman is pregnant with twins or triplets, a blood test will not be as reliable because the substances from a Down syndrome fetus may be harder to detect.

Prenatal Diagnostic Testing for Down Syndrome

If a screening test suggests the likelihood of Down syndrome, a diagnostic test can be performed. ACOG recommends that pregnant women of all ages be given the option of skipping the screening test and getting a diagnostic test first. Until recently, only women over age 35 and other at-risk women were offered this option because diagnostic tests carry a slight risk of miscarriage. Before having diagnostic testing, a pregnant woman and her family may want to meet with a health care provider to discuss their family history and the risks and benefits of testing in their specific situation.
Diagnostic testing for Down syndrome involves removing a sample of genetic material. After it is removed, the sample is checked for extra material from chromosome 21, which may indicate that a fetus has Down syndrome. The following procedures are used to extract samples.
Prenatal diagnostic testing does involve some risk to the mother and fetus, including risk of miscarriage that ranges from less than 1% to 2%.6,7,8,9 If you and your family are considering prenatal diagnostic testing for Down syndrome, discuss all the risks and benefits with your health care provider.
Chromosomal Testing of Maternal Blood
A pregnant woman who is at risk for having an infant with Down syndrome also can have a chromosomal test using her blood. A mother's blood carries DNA from the fetus, which may show extra chromosome 21 material.3 A more invasive test then would usually confirm the blood test.
Testing and In Vitro Fertilization
Another approach to diagnosis is used in conjunction with in vitro fertilization. Preimplantation genetic diagnosis (PGD) allows clinicians to detect chromosome imbalances or other genetic conditions in a fertilized egg before it is implanted into the uterus.
This technique is useful mostly for couples who are at risk of passing on a variety of genetic conditions, including X-linked disorders, as well as couples who have suffered repeated spontaneous pregnancy losses, sub-fertile couples, or those at risk for single-gene disorders.
Those interested in PGD should have genetic counseling and should consider close monitoring and additional testing during their pregnancies, given some increased risk of chromosomal abnormalities arising secondary to the in vitro fertilization process.

Reference: National Institutes of Child Health and Human Development