No test is 100% accurate. A screening test may be negative even when the baby has a birth defect. This is called a false-negative test result. It also is possible that a test will be positive-meaning the test result is abnormal-but the baby does not have the problem. This is called a false-positive test result.

A screening test is considered positive when the results show that your chance of having a child with a birth defect is the same as, or higher than, that of a 35-year-old woman. When a screening test is positive, a chorionic villus sampling or amniocentesis will likely be recommended to find out for sure whether the baby has a problem.

Ultrasound accuracy has been reported to be:³

• 53% overall for finding birth defects.
• 90% for finding fatal defects.
• About 100% for some neural tube defects, such as anencephaly.

Chorionic villus sampling or amniocentesis have an accuracy of more than 99% for Down syndrome.⁴