What is invasive prenatal diagnosis?

Invasive testing is a procedure that requires insertion of a needle by a doctor, under ultrasound control, to take a sample of placenta (chorionic villus sampling, CVS), amniotic fluid surrounding the baby (amniocentesis) or (rarely nowadays) of the baby's blood (cordocentesis). These techniques are usually performed with you wide awake and a fine needle is inserted through your abdomen often with some local anaesthetic so that you shouldn't feel anything. CVS is normally performed after 11, amniocentesis after 15 and cordocentesis after 18 weeks and with all techniques it is possible to 'culture' the fetal cells obtained allowing chromosomal or genetic diseases to be picked up. Rapid results are now available within a few days using techniques such as FISH (fluorescent in situ hybridisation) and PCR (polymerase chain reaction), however results may take as long as three weeks to arrive (amniocentesis) but are quicker with CVS and cordocentesis. These methods are the only way to give a certain diagnosis of your baby but very rarely, the cells obtained don't grow and therefore don't give an answer, or there is so much contamination of the sample by your own cells that no diagnosis is possible.
Amniocentesis: Guiding the needle

First, to help the doctor be absolutely precise when inserting the needle, an ultrasound scanner is used to establish the exact position of the fetus and the placenta. Second a fine needle is inserted into the womb and into a pool of amniotic fluid.

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