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A marker and detail ultrasound is performed to look for specific conditions in the fetus that are known to be related to chromosomal abnormalities. A marker is a slight deviation from the normal anatomy. Markers may or may not indicate a problem. An amniocentesis or chorionic villus sampling (CVS) would be used for further diagnosis and to confirm chromosomal abnormalities. It is typically done when you are 16 weeks pregnant.