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How does an ultrasound help diagnose a baby with Down syndrome?

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An ultrasound cannot diagnose whether a fetus has Down syndrome, but it can find characteristics called soft markers that may indicate the possibility of Down syndrome, reports About.com. Doctors must perform other prenatal diagnostic tests to verify that the fetus has the chromosome abnormalities associated with Down syndrome.

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Soft markers indicating increased risk of Down syndrome include a small or absent nose bone, thickness in the back of the neck, an abnormality in the artery leading to the upper extremities, and dilated brain ventricles, explains Medical News Today. Other markers include shorter arm or thigh bones, swelling of the kidneys, and bright spots in the bowels or heart. However, ultrasounds are only screening tests, and most fetuses that show such markers are healthy, notes About.com.

Doctors also perform blood tests during pregnancy as part of the screening process for Down syndrome, according to Mayo Clinic. A number of diagnostic tests can confirm indications of ultrasound soft markers, but they all carry various risks of miscarriage. In amniocentesis, doctors use a needle to withdraw amniotic fluid and use it to analyze the fetus's chromosomes. Chorionic villus sampling analyzes fetal chromosomes from the cells of the placenta. In cordocentesis, doctors withdraw and examine blood from the umbilical cord.

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