Performed in the first trimester of pregnancy, a PreNatest (prenatal screening test) uses a drop of blood to assess the risk of carrying a fetus with one of the most common birth abnormalities: Trisomy 21 (Down Syndrome) and Trisomy 18. This test does not provide a definitive diagnosis for fetal chromosome abnormalities. In combination with the 1st trimester ultrasound and a nuchal translucency scan, it is used primarily to identify cases where a more invasive analysis such as amniocentesis (direct analysis [karyotype] of chromosomes in the amniotic fluid) would be indicated. PreNatest Plus categorizes the risk as low (less than a 1 in 2500 chance of having a fetus with Trisomy 21 or 18), intermediate (1 in 251 to 1 in 2500 chance) or high (1 in 250 chance). For the intermediate and high risk categories, another non-invasive test (2nd trimester PreNatest) is first proposed. Based on the results of this second test, the physician or genetic counsellor can discuss available options.