The risk of giving birth to a child with a birth defect does increase as the mother’s age increases. The most common of these disorders is Down syndrome, a combination of mental retardation and physical abnormalities caused by the presence of an extra chromosome 21 (humans have 23 pairs of chromosomes). At age 25, a woman has ab out a 1-in-1,250 chance of having a baby with Down syndrome; at age 30, a 1-in-1,000 chance; at age 35, a 1-in-400 chance; at age 40, a 1-in-100 chance; and at 45, a 1-in-30 chance. Ask your doctor ab out tests that can help identify these types of problems. Blood tests are available to screen for some genetic disorders. Genetic amniocentesis or chorionic villus sampling is r out inely offered to women after the age of 35. Chorionic villus sampling can be done earlier in pregnancy and involves the removal of a minute amount of placental tissue, which can then be tested for genetic abnormalities. Amniocentesis is usually done in the third or fourth month of pregnancy and involves removal of amniotic fluid from the womb for genetic testing. There is a minimal (0.5%) risk of miscarriage associated with this test. Ab out 95 percent of women who undergo prenatal testing receive the reassuring news that their baby does not have one of these disorders.