The first widely available postconception test was for Down syndrome. This condition becomes more common with increasing age of the pregnant woman, reaching a level of 1 per 11 births at a maternal age of 48. Because testing for Down syndrome requires amniocentesis, with its attendant costs and risks, in women without specific risk factors the test is recommended only for those over 34 years of age. This is an arbitrary figure, however, because the risk rises relatively smoothly from the age of 21. As a result, approximately 80% of cases of Down syndrome occur in babies born to women under age 35. Once most pregnant women were being screened for MSAFP, it was found that an abnormally low level of MSAFP indicates an increased probability of Down syndrome. This information can be used to identify women under age 35 who should be offered amniocentesis for potential Down syndrome. [ACOG Technical Bulletin 154. Prenatal Detection of Neural Rube Defects. April 1991.]