Definition
First trimester screening is a relatively new prenatal test that offers early information about a baby's health. Typically, first trimester screening is done between weeks 11 and 14 of pregnancy — earlier than many other prenatal tests.
First trimester screening includes two steps: a blood test to measure levels of two pregnancy-specific substances and a specialized ultrasound exam to measure the size of your baby's nuchal fold — a specific area at the back of your baby's neck. Using your age and the results of the blood test and the ultrasound, your health care provider can gauge your risk of carrying a baby who has Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18). Down syndrome causes mental retardation. Edwards syndrome causes more severe retardation.
If your risk level is low, first trimester screening can offer reassurance of a healthy pregnancy. If your risk level is moderate or high, you may choose to follow first trimester screening with more invasive diagnostic testing — such as chorionic villus sampling or amniocentesis — to determine whether the baby actually has Down syndrome or Edwards syndrome.
Source:
http://www.mayoclinic.com
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