the procedure itself. The age cutoff used varies somewhat among different prenatal genetics centers but is usually at least 3 1 to 32 years of age 2. Previous child with a de novo chromosome abnormality: If the parents of a child with a chromosome abnormality have normal chromosomes themselves there may nevertheless be a risk of the same abnormality in a subsequent child. For example, if a woman under 30 years of age has a child with Down syndrome, the recurrence risk is about 1/100, in comparison with a general population risk of about 1/800. Prenatal mosaicism is possible exp lanation ofthe increased risk. 3. Presence of structural chromosome abnormality in one of the parents Here the risk of an abnormal child is usually 20% or less, but it may be higher 4. Family history of some genetic defect that may be diagnosed or ruled out by biochemical or DNA analysis: Most of these disorders are caused by single-gene defects and have risks of 25% or 50% in sibs of affected children. Cases in which the parents have been diagnosed as carriers after a population screening test rather than after the birth of an affected child are also in this category. Even before DNa analysis entered the picture numerous biochemical disorders could be identified prenatally, and dna analysis has greatly increased the number 5. Family history of an X-linked disorder for which there is no specifi prenatal diagnostic test: When there is no alternative method, the parents may use fetal sex determination to help them decide whether to continue or terminate the pregnancy. With the development of dNA analysis prenatal diagnosis of X-linked disorders such as Duchenne muscular dystrophy and hemophilia A and B, first the fetal sex is determined, and then dna analysis is performed if the fetus is male 6. Risk of a neural tube defect (NTD): Only first-and second-degree relatives of ntd patients are eligible for amniocentesis because of a significantlythe procedure itself. The age cutoff used varies somewhat among different prenatal genetics centers but is usually at least 31 to 32 years of age. 2.Previous child with a de novo chromosome abnormality: If the parents of a child with a chromosome abnormality have normal chromosomes themselves, there may nevertheless be a risk of the same abnormality in a subsequent child. For example, if a woman under 30 years of age has a child with Down syndrome, the recurrence risk is about 1/100, in comparison with a general population risk of about 1/800. Prenatal mosaicism is possible explanation of the increased risk. 3.Presence of structural chromosome abnormality in one of the parents: Here the risk of an abnormal child is usually 20% or less, but it may be higher. 4.Family history of some genetic defect that may be diagnosed or ruled out by biochemical or DNA analysis: Most of these disorders are caused by single-gene defects and have risks of 25% or 50% in sibs of affected children. Cases in which the parents have been diagnosed as carriers after a population screening test rather than after the birth of an affected child are also in this category. Even before DNA analysis entered the picture, numerous biochemical disorders could be identified prenatally, and DNA analysis has greatly increased the number. 5.Family history of an X-linked disorder for which there is no specific prenatal diagnostic test: When there is no alternative method, the parents may use fetal sex determination to help them decide whether to continue or terminate the pregnancy. With the development of DNA analysis for prenatal diagnosis of X-linked disorders such as Duchenne muscular dystrophy and hemophilia A and B, first the fetal sex is determined, and then DNA analysis is performed if the fetus is male. 6.Risk of a neural tube defect (NTD): Only first-and second-degree relatives of NTD patients are eligible for amniocentesis because of a significantly