Amniocentesis is used by doctors to help diagnose a number of birth defects during the second trimester at the slight risk of inducing miscarriage or causing an infection. MedlinePlus lists several disorders that can be identified by way of amniocentesis and describes the procedure in detail.
Amniocentesis is the process of extracting a small amount of amniotic fluid. According to MedlinePlus, this is approximately 4 teaspoons of fluid to be tested for abnormalities. After a blood test and an ultrasound examination, a long, thin needle is inserted into the mother's abdomen until it reaches the protective membrane around the fetus. Fluid is then extracted and sent for analysis.
According to WebMD, this test is helpful for diagnosing potentially serious health issues for the developing fetus including Down syndrome, sickle-cell anemia and muscular dystrophy. It is most often done for mothers who have a family history of such disorders, demonstrated an abnormal ultrasound, experienced previous complications related to childbearing or are over the age of 35 during their pregnancy.
WebMD also explains some of the risks of amniocentesis. Amniocentesis carries a risk of between 0.5 percent and 0.25 percent of inducing miscarriage. There is also a slight chance of preterm labor, infection or injury to the fetus caused by the needle, but these cases are extremely rare.