First trimester combined nuchal translucency and biochemical screening (OSCAR)

1. What is OSCAR?

  • OSCAR is a mnemonic and stands for One Stop Clinic for Assessment of Risk for Downs syndrome. In many countries, it is named ‘first trimester combined nuchal translucency (NT) and biochemical screening’;
  • It is a screening test performed at 11-13 6/7 weeks of gestation, mainly for Downs syndrome. It can also screen for Edwards and Patau syndrome
  • The following parameters are obtained to calculate the risks of having a baby affected by Downs, Edwards or Patau syndrome for each individual woman:
  1. (i) The woman’s age;
  2. (ii) Her serum level of pregnancy-associated plasma protein-A (PAPP-A);
  3. (iii) Her serum level of free beta human chorionic gonadotrophin (free-beta hCG);
  4. (iv) Thickness of the baby’s neck-fold (nuchal translucency thickness)

2. How is OSCAR performed? 

  • Ultrasound for measuring the NT thickness of the fetus;
  • Taking blood from the mother for the hormonal levels;
  • All have to be performed at 11-13 6/7 weeks of gestation;
  • A computer software will calculate the risks for Downs, Edwards and Patau syndromes based on the maternal age, NT and the levels of the hormones 

3. How accurate is the test?

  • The detection rates for Downs, Edwards and Patau syndromes are ~90%; 
  • There is a false positive rate of ~5%, meaning that 5% of all women with normal babies will have a positive result

4. What if the OSCAR results are abnormal?

  • Not all women with abnormal results have abnormal babies. Approximately 5% of all women will have an abnormal result;
  • The reasons leading to an increase in risk can be contributed by the woman’s age, the increase in NT, abnormal hormonal profile or a combination of any of the above. The implication can be different;   
  • It is important to discuss with a specialist in this field to determine the next appropriate option, whether it is NIPTchorionic villus sampling or amniocentesis

5. What other abnormalities can OSCAR screen for?

  • NT measurement is an essential part of the OSCAR test; there is a normal range of NT for different fetal size;
  • Increase in NT is an important marker not just for fetal chromosomal abnormalities, but also for fetal structural defects, in particular congenital heart diseases. NT is also a good screening test for some genetic conditions

6. How is OSCAR compared with other antenatal screening tests for Downs syndrome?



Mid-trimester (16-20 weeks)

biochemical screening


Detection rate




False positive rate





This article is contributed by Dr. T.N. Danny Leung
Updated on 25.09.2020