MODERN GENETIC ANALYSIS by Anthony J. F. Griffiths, William M. Gelbart, Jeffrey H. Miller, and Richard C. Lewontin ©1999, 2002 by W.H. Freeman and Company. Used with permission.

Performed usually at sixteen weeks or later, amniocentesis is a test used to determine if there are any genetic defects in the baby. Guided by ultrasound, a long needle is inserted into the uterus and fluid is extracted. The fluid is then evaluated by a laboratory for certain defects, infections, and/or fetal maturity.

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