Ok, ladies - did some research:
http://www.fetalmedicine.hk/en/oscardetails.asp
OSCAR stands for "One-Stop Clinic for Assessment of Risk". This is the latest prenatal screening program for Down's Syndrome, introduced from Fetal Medicine Foundation of United Kingom. The screening program is performed during 11-14 weeks of gestation, and consists of the measurements of Nuchal Translucency, maternal serum level of Pregnancy Associated Plasma Protein-A (PAPP-A), and free beta-human Chorionic Gonadotropin (free beta-hCG). The chance of having a Down fetus is computed using the above information. This screening test is safe with a detection rate of 90%.
Procedures
Firstly, ultrasound scanning is performed to confirm the gestational age and fetal size. The nuchal translucency is also measured.
Blood is then taken from the pregnant woman for PAPP-A and free-beta hCG assay. The result will be available after an hour.
A software issued by Fetal Medicine Foundation will be used to compute the chance of Down's syndrome. The test is regarded as positive if the computed chance is higher than 1:300; otherwise it is negative.
About 7% of all women undergoing the screening will have a positive result (high risk). Further invasive procedures such as chorionic villus sampling or amniocentesis will be required to confirm whether the fetus a Down's. We recommend chorionic villus sampling as it can be performed before 14 weeks, and have the same risk of amniocentesis (1% abortion rate).
A negative screening test indicate that the chance of having a Down fetus is very low, though it cannot be totally excluded. Invasive procedures are not indicated.