Due to the greater sensitivity and specificity in diagnosing chromosome abnormalities, prenatal diagnosis is always recommended. Options include: Chorionic villi sampling (CVS), which is the withdrawal of a small amount of chorionic tissue (tissue from the developing placenta). CVS is performed between 10 and 12 weeks of pregnancy. Chromosome analysis is conducted on the sample. Amniocentesis, which is the withdrawal of a small amount of amniotic fluid from the uterus. The fluid can be tested for chromosome abnormalities and for open neural tube defects. Typically an amniocentesis is performed around the 16th week of pregnancy.