Amniocentesis is a diagnostic procedure in medical genetics used for prenatal testing of a fetus. A sample of the amniotic
fluid that bathes the human fetus is removed from the pregnant uterus by suction with a very fine needle. Fetal cells suspended in the fluid can then be grown in the laboratory and studied to detect various genetic disorders in the unborn child.
Diagnostic
amniocentesis is usually performed during the second trimester of pregnancy, thus giving ample time to complete the appropriate studies, and, if necessary, permit therapeutic abortion when it is still relatively safe.
Amniocentesis may be advised in several situations:
Chromosome Abnormalities
The risk of Down syndrome (a mental deficiency associated with an extra chromosome 21 in the cells) and other chromosomal disorders increases with the age of the mother and with a history of previously affected offspring.
The risk of bearing a child with Down syndrome is roughly 1 in 1,500 women in their 20s, 1 in 300 women in their 30s, and 1 in 40 women in their 40s. Therefore, amniocentesis has been advised for women over the age of 35 and for those who previously have been found to have chromosomal abnormalities.
Biochemical Disorders
Frequently, a child of apparently normal parents is born with an inherited biochemical disorder. Genetic testing of the parents may indicate the possibility of a significant recurrent risk in future offspring, and in many cases amniocentesis can then be performed in later pregnancies to determine whether the developing fetus is normal.
Sex-Linked Disorders
Many sex-linked disorders existÑfor example, hemophilia, a blood-coagulation deficiency, and the Duchenne form of muscular dystrophy, both of which affect only males, and _fragile X syndrome, which primarily and more severely affects males. Until recently, the genetic defects that cause these disorders were unknown, so that amniocentesis could be used only to learn the sex of the developing fetus. Now, however, direct genetic analysis of sex-linked disorders is possible.
Congenital
Malformations Such congenital malformations of the brain and spinal cord as anencephaly and meningomyelocele are associated with increased levels of alpha-fetoprotein, a substance in the amniotic fluid. Since the risk of recurrent congenital malformations is higher in some families than in the general population, a woman who has given birth to an affected child might seek prenatal diagnosis.