Amniocentesis samples amniotic fluid. Karyotyping involves growing fetal cells that are shed into amniotic fluid. DNA tests for specific disorders (such as cystic fibrosis, myotonic dystrophy, and fragile X syndrome) is done on amniocentesis derived material. Amniotic fluid is largely fetal urine, lacking waste products that are removed from the circulation in utero by the placenta. Amniotic fluid can be analyzed for hormones, toxins, metabolites, drugs and trace elements. Amniotic fluid can be cultured or analyzed by light or electron microscopy for evidence of infectious agents.
One of the routine uses for an amnio is for determining the functional maturity of the lungs in the third trimester when there is a risk or necessity for premature delivery. The lungs are ready to work, in air, when 'surfactant' is produced in sufficient amount by the cells lining the air sacs of the lungs. This happens normally in the last month of gestation. Amniotic fluid can be assayed for surfactants, including counting the 'lamellar' bodies, which are the physical form of surfactant. Surfactant tends to be produced a bit earlier in girls than in boys and when there is stress, delayed production occurs with maternal pre-pregnancy diabetes. Artificial surfactant is given to infants who are delivered before functional maturation of the lungs is complete.
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