When a screening test turns out positive, your obstetrician will recommend more accurate diagnostic test. There are 2 main prenatal test current.
A small needle is guided into the amniotic sac to draw out some amniotic fluid. The fluid is then used to culture fetal cells. These are tested for any abnormalities in the DNA. This can be only done from 15 weeks of gestation onwards. The main risk is that of miscarriage and is about 1%. Other risk includes not being able to culture fetal cells.
Chorionic Villus Sampling
This is done by using a small needle to withdraw some cells from the placenta. This is done with ultrasound guidance and can be done trans-abdominally or trans-cervically.
The test can be done around 10 to 13 weeks of gestation. The risk if miscarriage is about 1% but purportedly slightly higher than amniocentesis.