Prenatal DNA Testing
Determining the paternity of a fetus or assessing the risks of the child being born with Down syndrome or other health-linked gene abnormality are the most common reasons for prenatal DNA testing, although such testing does not routinely have to be done before the child's birth. The decision to perform testing while the child is still in the womb is one that should be made by the parents with input from their doctor and possibly a genetic counselor.

Most fetal gene testing is invasive, therefore not without risk to the mother and baby. There are three common methods for DNA testing: amniocentesis, chorionic villus (placental) sampling and umbilical blood sampling. Each involves the insertion of a needle into the placenta or umbilical cord for withdrawal of fetal cells, thus presenting an opportunity for a miscarriage. The test used is partially determined by which trimester the pregnancy is currently in.

New testing methods have been developed which are less invasive and carry less risk. Because cells from the fetus can pass through the placenta and into the mother's bloodstream, these test methods use the mother's blood to obtain fetal DNA cells for additional information.

There are two tests performed on maternal blood that are helpful in determining gender, paternity and/or gene abnormalities that could result in birth defects such as Down syndrome or spina bifida.

One is the Triple screen test, which tests the levels of three substances in the mother's blood that could indicate abnormalities in the fetal DNA. Another, the Quad screen test, tests the levels of an additional substance, a particular hormone, to obtain similar information. These tests can be supplemented with a detailed ultrasound, or sonogram, which can provide visual information about any structural abnormalities which may be present.

Testing information is combined with information about the mother - her age, weight, race and health history - to predict as accurately as possible the likelihood of congenital abnormalities. There is always a possibility of false positive and false negative results, however.

Further testing can be done if initial tests indicate the possibility of abnormalities.