An amniocentesis, or amnio for short, is usually done between 15 to 20 weeks of pregnancy and involves using a narrow needle inserted into the mother's abdomen to remove a small sample of amniotic fluid from around the fetus. This amniotic fluid contains fetal skin cells that have been sloughed off. The fluid is sent to a cytogenetics laboratory for chromosome analysis either by FISH analysis or by karyotyping. Chromosome analysis will reveal if the fetus has Down syndrome. There is a small risk (less than one in 400) that an amniocentesis can cause a miscarriage.