Amniocentesis, typically performed between 15-18 weeks gestational age, can detect neural tube defects, such as spina bifida or anencephaly. An ultrasound will detect defects later in the pregnancy and can confirm the diagnosis of a neural tube defect at 18-20 weeks gestational age. Chorionic villus sampling (CVS) is used to screen the fetus for genetic disorders but cannot diagnose neural tube defects. Carrier screening is a genetic test taken before conception that determines the risk that certain genetic diseases (if any) the prospective parents might pass on to their offspring.