For expectant mothers > 35 years old, skip OSCAR Test

If you're an expectant mother above age 35 (especially above 40) on the estimated birth date of your child, skip the OSCAR prenatal test. It's a waste of money and, most importantly, causes unnecessary emotional stress and anxiety to you and your family. Go direct for the more detailed and confirmative prenatal tests, such as Amniocentesis, CVS or Harmony instead. Otherwise, simply skip both tests if you would keep your baby regardless of the outcome.

What's OSCAR test?

It stands for "One-Stop Clinic for Assessment of Risk". From the ultrasound scan and blood test, the OSCAR takes in four different markers and compute your probability risks in your pregnancy. There are 3 types of risks, namely Trisomy 21 (T21), Trisomy 18 (T18)  and Trisomy 13 (T13). In most simplistic terms, T18 and T13 are related to unhealthy development of baby's organs and are likely lead to miscarriage and still-born (i.e. Edwards and Patau's syndromes).  T21 is related to Down's Syndrome (DS) and is the most likely indicator to raise a red flag.

In short, OSCAR test is meant to determine the probability risk of your pregnancy and is conducted towards the end of first trimester (12-13 weeks). To compute the probability risk, there are four markers that are input parameters to the computer program. After searching the Internet extensively for information, these are what I have gathered so far:

1. Expectant age: risk increases exponentially from age >35
2. NT measurement (thickness of neck): should be < 3mm. Ideally < 2.5mm. This is an important marker.
3. Free beta hCG: should be within 0.4 - 2.5 MoM. Ideally as close as 1MoM as possible. < 0.4 MoM increases the risk of T21 while >2.5MoM increases the risk of T18 and T13
4. PAPP-A: should be > 0.4 MoM. Ideally as close as 1 MoM as possible. < 0.4 MoM increases the risk of T21. There is no known adverse finding for high PAPP-A score. 

Both PAPP-A and hCG should be read together. T21 are generally associated with extremely low PAPP-A and high hCG. Both extremely low PAPP-A and hCG (i.e. both < 0.4 MoM) may indicate strong likelihood in T18 and T13. Another very important indicator is the detection of Nasal Bone (NB) presence during the ultrascan. The absence of NB or very short length of NB could indicate high possibility in DS. Check out this useful site for more detailed understanding of these markers. And another site to understand the acceptable ranges in these markers.

If you're considered a high risk mother (> 1:300 chance), a more expensive and detailed test, such as Amniocentesis, CVS or Harmony, is recommended. In a way, OSCAR is designed to save you time and money.

Problem in OSCAR

But there is a problem. The computer software used in determining the exact probability risk is questionable. I'm not sure exactly how it works in detail. In high level term as discussed above, it takes in your age, NT measurement, free beta hCG and PAPP-A, and then compute a probability.

In other words, you may get different results if you go to different clinics (also mean different computer programs). You can try out this online Trisomy Risk Calculator to have a good feel in how the Trisomy computer program works. In fact, the probability score obtained from this site can be miles apart from the actual scores obtained from different clinics.

Trisomy Risk Calculator (TCR), is a free online tool that demonstrates and confirms the huge potential of prenatal screening for Down's, Edwards and Patau's syndromes, which can be used to contrast with calculations currently provided by specialized centers.

OSCAR computation takes in age factor as key besides NT scan result. If you're already >35 (especially above 40), your age specific risk (or background risk) is already not in your favor (> 1:300 risk threshold). Most importantly, OSCAR is not affirmative. It only tells you a probability.

Why not just go for a more expensive and affirmative test directly?  Many mothers in online forums decried its reliability and claimed it would only cause undue emotional anxiety and stress during the wait (can be several weeks apart between a bad OSCAR probability score and the affirmative test result). Try google "Bad Oscar result". Furthermore, OSCAR is not exactly cheap @ ~S$460 Thomson Medical Centre, Singapore.

The Good News

In real world, the chance of delivering a healthy baby eventually is overwhelmingly in your favor, even if you were considered a high risk expectant mother. In fact, the risk of miscarriage is higher than delivering a Down's Syndrome baby. Unhealthy fetus is more likely to go away on its own.