The same AFP protein is found in relatively low levels in a foetus that has the Trisomy 21 defect.  The AFP marker in conjunction with information on the age and ethnicity of the parents can help determine a 'high risk' Trisomy 21 pregnancy.

The HCG marker (Human Chorionic Gonadotropin).  As mentioned in the article B-HCG Levels in Pregnancy the levels of ß-hCG should drop away 8-9 weeks (from conception) into the pregnancy as the placenta takes over production of progesterone.  A foetus with Trisomy 21 produces high levels of b-hCG, so if higher than expected levels of ß-hCG are found between 15 and 22 weeks, then this combined with information about the mother may indicate a Down Syndrome foetus.

The UE3 marker (Unconjugated Estriol) is another hormone produced by the foetus and placenta but is not used to specifically indicate an anomaly, rather it is used as a reference against the other two markers to increase the accuracy of the information provided thus reducing the number of false results (details on results follow).

Tests Results:  Remember, this test is not a diagnosis, but a screening, this means that the results that are produced indicate only the risk that the mother has of carrying a genetically disordered foetus.  

The results are compared to age and ethnicity, for example, the result may say that a white North European couple where the woman is under 25 years old and has these levels of the three hormones at 16 weeks (14 from conception) has a chance of 1 in 10,000 of having a Trisomy 21 child.  Alternatively it may say that the chances are 1 in 200. In either case, it is just a chance and not an absolute.

Abnormal test results do not automatically indicate an abnormal foetus.  In fact most women that have an abnormal/positive first test find that the test is proved false.  

Often, the normal course of action for obstetricians on seeing a positive test result with the Triple Test is to recommend further testing with Amniocentesis or CVS. However in many places in the world a more cautious approach is adopted by ordering a second triple test.  If a second positive is found then a high definition ultrasound is performed to check on gestational age or multiple pregnancy and only if this shows that accurate information has been used will an invasive procedure be recommended.  

The options for invasive testing should be discussed in detail with both your medical practitioner or counselor and between each other due to the higher risks of damage to the foetus involved in these procedures.

Introduction

1. The Triple Test - Overview

2. The Triple Test - Detail and Results

3. Amniocentesis

4. CVS